diff --git a/lib/email/receiver.rb b/lib/email/receiver.rb
index e143c82b753..57061de3faa 100644
--- a/lib/email/receiver.rb
+++ b/lib/email/receiver.rb
@@ -239,7 +239,7 @@ module Email
 
       if text.present?
         text = trim_discourse_markers(text)
-        text, elided_text = EmailReplyTrimmer.trim(text, true)
+        text, elided_text = trim_reply_and_extract_elided(text)
 
         if @opts[:convert_plaintext] || sent_to_mailinglist_mirror?
           text_content_type ||= ""
@@ -255,7 +255,7 @@ module Email
       markdown, elided_markdown = if html.present?
         markdown = HtmlToMarkdown.new(html, keep_img_tags: true, keep_cid_imgs: true).to_markdown
         markdown = trim_discourse_markers(markdown)
-        EmailReplyTrimmer.trim(markdown, true)
+        trim_reply_and_extract_elided(markdown)
       end
 
       if text.blank? || (SiteSetting.incoming_email_prefer_html && markdown.present?)
@@ -265,6 +265,11 @@ module Email
       end
     end
 
+    def trim_reply_and_extract_elided(text)
+      return [text, ""] if @opts[:skip_trimming]
+      EmailReplyTrimmer.trim(text, true)
+    end
+
     def fix_charset(mail_part)
       return nil if mail_part.blank? || mail_part.body.blank?
 
diff --git a/script/import_scripts/base.rb b/script/import_scripts/base.rb
index 4d177156b40..7f683818f7a 100644
--- a/script/import_scripts/base.rb
+++ b/script/import_scripts/base.rb
@@ -31,6 +31,7 @@ class ImportScripts::Base
     @site_settings_during_import = {}
     @old_site_settings = {}
     @start_times = { import: Time.now }
+    @skip_updates = false
   end
 
   def preload_i18n
@@ -46,14 +47,16 @@ class ImportScripts::Base
 
     puts ""
 
-    update_bumped_at
-    update_last_posted_at
-    update_last_seen_at
-    update_user_stats
-    update_feature_topic_users
-    update_category_featured_topics
-    update_topic_count_replies
-    reset_topic_counters
+    unless @skip_updates
+      update_bumped_at
+      update_last_posted_at
+      update_last_seen_at
+      update_user_stats
+      update_feature_topic_users
+      update_category_featured_topics
+      update_topic_count_replies
+      reset_topic_counters
+    end
 
     elapsed = Time.now - @start_times[:import]
     puts '', '', 'Done (%02dh %02dmin %02dsec)' % [elapsed / 3600, elapsed / 60 % 60, elapsed % 60]
diff --git a/script/import_scripts/mbox/importer.rb b/script/import_scripts/mbox/importer.rb
index d894b24f9eb..3886f33bb69 100644
--- a/script/import_scripts/mbox/importer.rb
+++ b/script/import_scripts/mbox/importer.rb
@@ -24,9 +24,14 @@ module ImportScripts::Mbox
 
     def execute
       index_messages
-      import_categories
-      import_users
-      import_posts
+
+      if @settings.index_only
+        @skip_updates = true
+      else
+        import_categories
+        import_users
+        import_posts
+      end
     end
 
     def index_messages
diff --git a/script/import_scripts/mbox/settings.yml b/script/import_scripts/mbox/settings.yml
index 942ee708d4e..a4b449a3d6f 100644
--- a/script/import_scripts/mbox/settings.yml
+++ b/script/import_scripts/mbox/settings.yml
@@ -1,11 +1,18 @@
-# PostgreSQL mailing lists
-#data_dir: /shared/import/data
-#split_regex: "^From .*@postgresql.org.*"
-
-# ruby-talk mailing list
 data_dir: /shared/import/data
-split_regex: ""
+
+# mbox files
+split_regex: "^From .+"
+#split_regex: "^From .+@example.com.+"
+
+# individual emails
+#split_regex: ""
+
+# Listserv files
+#split_regex: "^========================================================================="
 
 default_trust_level: 1
 prefer_html: false
 staged: true
+index_only: false
+
+group_messages_by_subject: false
diff --git a/script/import_scripts/mbox/support/database.rb b/script/import_scripts/mbox/support/database.rb
index 2a20960c25b..085dd2f9139 100644
--- a/script/import_scripts/mbox/support/database.rb
+++ b/script/import_scripts/mbox/support/database.rb
@@ -2,7 +2,7 @@ require 'sqlite3'
 
 module ImportScripts::Mbox
   class Database
-    SCHEMA_VERSION = 1
+    SCHEMA_VERSION = 2
 
     def initialize(directory, batch_size)
       @db = SQLite3::Database.new("#{directory}/index.db", results_as_hash: true)
@@ -17,6 +17,15 @@ module ImportScripts::Mbox
       create_table_for_users
     end
 
+    def transaction
+      @db.transaction
+      yield self
+      @db.commit
+
+    rescue
+      @db.rollback
+    end
+
     def insert_category(category)
       @db.execute(<<-SQL, category)
         INSERT OR REPLACE INTO category (name, description)
@@ -35,10 +44,10 @@ module ImportScripts::Mbox
       @db.execute(<<-SQL, email)
         INSERT OR REPLACE INTO email (msg_id, from_email, from_name, subject,
           email_date, raw_message, body, elided, format, attachment_count, charset,
-          category, filename, first_line_number, last_line_number)
+          category, filename, first_line_number, last_line_number, index_duration)
         VALUES (:msg_id, :from_email, :from_name, :subject,
           :email_date, :raw_message, :body, :elided, :format, :attachment_count, :charset,
-          :category, :filename, :first_line_number, :last_line_number)
+          :category, :filename, :first_line_number, :last_line_number, :index_duration)
       SQL
     end
 
@@ -69,7 +78,21 @@ module ImportScripts::Mbox
       SQL
     end
 
-    def sort_emails
+    def update_in_reply_to_by_email_subject
+      @db.execute <<-SQL
+        UPDATE email
+        SET in_reply_to = NULLIF((
+          SELECT e.msg_id
+          FROM email e
+            JOIN email_order o ON (e.msg_id = o.msg_id)
+          WHERE e.subject = email.subject
+          ORDER BY o.ROWID
+          LIMIT 1
+        ), msg_id)
+      SQL
+    end
+
+    def sort_emails_by_date_and_reply_level
       @db.execute 'DELETE FROM email_order'
 
       @db.execute <<-SQL
@@ -90,6 +113,17 @@ module ImportScripts::Mbox
       SQL
     end
 
+    def sort_emails_by_subject
+      @db.execute 'DELETE FROM email_order'
+
+      @db.execute <<-SQL
+        INSERT INTO email_order (msg_id)
+        SELECT msg_id
+        FROM email
+        ORDER BY subject, filename, ROWID
+      SQL
+    end
+
     def fill_users_from_emails
       @db.execute 'DELETE FROM user'
 
@@ -164,10 +198,17 @@ module ImportScripts::Mbox
 
     def configure_database
       @db.execute 'PRAGMA journal_mode = OFF'
+      @db.execute 'PRAGMA locking_mode = EXCLUSIVE'
     end
 
     def upgrade_schema_version
-      # current_version = query("PRAGMA user_version").last[0]
+      current_version = @db.get_first_value("PRAGMA user_version")
+
+      case current_version
+      when 1
+        @db.execute "ALTER TABLE email ADD COLUMN index_duration REAL"
+      end
+
       @db.execute "PRAGMA user_version = #{SCHEMA_VERSION}"
     end
 
@@ -211,11 +252,13 @@ module ImportScripts::Mbox
           filename TEXT NOT NULL,
           first_line_number INTEGER,
           last_line_number INTEGER,
+          index_duration REAL,
           FOREIGN KEY(category) REFERENCES category(name)
         )
       SQL
 
       @db.execute 'CREATE INDEX IF NOT EXISTS email_by_from ON email (from_email)'
+      @db.execute 'CREATE INDEX IF NOT EXISTS email_by_subject ON email (subject)'
       @db.execute 'CREATE INDEX IF NOT EXISTS email_by_in_reply_to ON email (in_reply_to)'
       @db.execute 'CREATE INDEX IF NOT EXISTS email_by_date ON email (email_date)'
 
diff --git a/script/import_scripts/mbox/support/indexer.rb b/script/import_scripts/mbox/support/indexer.rb
index 251975f07da..e50f74449e7 100644
--- a/script/import_scripts/mbox/support/indexer.rb
+++ b/script/import_scripts/mbox/support/indexer.rb
@@ -8,12 +8,12 @@ module ImportScripts::Mbox
     # @param settings [ImportScripts::Mbox::Settings]
     def initialize(database, settings)
       @database = database
-      @root_directory = settings.data_dir
+      @settings = settings
       @split_regex = settings.split_regex
     end
 
     def execute
-      directories = Dir.glob(File.join(@root_directory, '*'))
+      directories = Dir.glob(File.join(@settings.data_dir, '*'))
       directories.select! { |f| File.directory?(f) }
       directories.sort!
 
@@ -24,14 +24,21 @@ module ImportScripts::Mbox
       end
 
       puts '', 'indexing replies and users'
-      @database.update_in_reply_to_of_emails
-      @database.sort_emails
+      if @settings.group_messages_by_subject
+        @database.sort_emails_by_subject
+        @database.update_in_reply_to_by_email_subject
+      else
+        @database.update_in_reply_to_of_emails
+        @database.sort_emails_by_date_and_reply_level
+      end
+
       @database.fill_users_from_emails
     end
 
     private
 
     METADATA_FILENAME = 'metadata.yml'.freeze
+    IGNORED_FILE_EXTENSIONS = ['.dbindex', '.dbnames', '.digest', '.subjects']
 
     def index_category(directory)
       metadata_file = File.join(directory, METADATA_FILENAME)
@@ -54,7 +61,7 @@ module ImportScripts::Mbox
     end
 
     def index_emails(directory, category_name)
-      all_messages(directory, category_name) do |receiver, filename, first_line_number, last_line_number|
+      all_messages(directory, category_name) do |receiver, filename, opts|
         msg_id = receiver.message_id
         parsed_email = receiver.mail
         from_email, from_display_name = receiver.parse_from_field(parsed_email)
@@ -75,12 +82,15 @@ module ImportScripts::Mbox
           charset: parsed_email.charset&.downcase,
           category: category_name,
           filename: File.basename(filename),
-          first_line_number: first_line_number,
-          last_line_number: last_line_number
+          first_line_number: opts[:first_line_number],
+          last_line_number: opts[:last_line_number],
+          index_duration: (monotonic_time - opts[:start_time]).round(4)
         }
 
-        @database.insert_email(email)
-        @database.insert_replies(msg_id, reply_message_ids) unless reply_message_ids.empty?
+        @database.transaction do |db|
+          db.insert_email(email)
+          db.insert_replies(msg_id, reply_message_ids) unless reply_message_ids.empty?
+        end
       end
     end
 
@@ -102,12 +112,18 @@ module ImportScripts::Mbox
 
         if @split_regex.present?
           each_mail(filename) do |raw_message, first_line_number, last_line_number|
+            opts = {
+              first_line_number: first_line_number,
+              last_line_number: last_line_number,
+              start_time: monotonic_time
+            }
             receiver = read_mail_from_string(raw_message)
-            yield receiver, filename, first_line_number, last_line_number if receiver.present?
+            yield receiver, filename, opts if receiver.present?
           end
         else
+          opts = { start_time: monotonic_time }
           receiver = read_mail_from_file(filename)
-          yield receiver, filename if receiver.present?
+          yield receiver, filename, opts if receiver.present?
         end
 
         mark_as_fully_indexed(category_name, filename)
@@ -132,10 +148,12 @@ module ImportScripts::Mbox
       each_line(filename) do |line|
         line = line.scrub
 
-        if line =~ @split_regex && last_line_number.positive?
-          yield raw_message, first_line_number, last_line_number
-          raw_message = ''
-          first_line_number = last_line_number + 1
+        if line =~ @split_regex
+          if last_line_number > 0
+            yield raw_message, first_line_number, last_line_number
+            raw_message = ''
+            first_line_number = last_line_number + 1
+          end
         else
           raw_message << line
         end
@@ -163,7 +181,7 @@ module ImportScripts::Mbox
     end
 
     def read_mail_from_string(raw_message)
-      Email::Receiver.new(raw_message, convert_plaintext: true) unless raw_message.blank?
+      Email::Receiver.new(raw_message, convert_plaintext: true, skip_trimming: false) unless raw_message.blank?
     end
 
     def extract_reply_message_ids(mail)
@@ -210,16 +228,12 @@ module ImportScripts::Mbox
     end
 
     def ignored_file?(filename, checksums)
-      File.directory?(filename) || hidden_file?(filename) ||
-        metadata_file?(filename) || fully_indexed?(filename, checksums)
-    end
+      filename = File.basename(filename)
 
-    def hidden_file?(filename)
-      File.basename(filename).start_with?('.')
-    end
-
-    def metadata_file?(filename)
-      File.basename(filename) == METADATA_FILENAME
+      filename.start_with?('.') ||
+        filename == METADATA_FILENAME ||
+        IGNORED_FILE_EXTENSIONS.include?(File.extname(filename)) ||
+        fully_indexed?(filename, checksums)
     end
 
     def fully_indexed?(filename, checksums)
@@ -230,5 +244,9 @@ module ImportScripts::Mbox
     def calc_checksum(filename)
       Digest::SHA256.file(filename).hexdigest
     end
+
+    def monotonic_time
+      Process.clock_gettime(Process::CLOCK_MONOTONIC)
+    end
   end
 end
diff --git a/script/import_scripts/mbox/support/settings.rb b/script/import_scripts/mbox/support/settings.rb
index a1fc74d3d1e..703ec2220ad 100644
--- a/script/import_scripts/mbox/support/settings.rb
+++ b/script/import_scripts/mbox/support/settings.rb
@@ -13,6 +13,8 @@ module ImportScripts::Mbox
     attr_reader :trust_level
     attr_reader :prefer_html
     attr_reader :staged
+    attr_reader :index_only
+    attr_reader :group_messages_by_subject
 
     def initialize(yaml)
       @data_dir = yaml['data_dir']
@@ -21,6 +23,8 @@ module ImportScripts::Mbox
       @trust_level = yaml['default_trust_level']
       @prefer_html = yaml['prefer_html']
       @staged = yaml['staged']
+      @index_only = yaml['index_only']
+      @group_messages_by_subject = yaml['group_messages_by_subject']
     end
   end
 end