PERF: user imports would slow down the more users were imported

script/import_scripts/base.rb

@@ -390,7 +390,8 @@ class ImportScripts::Base
   end
 
   def find_existing_user(email, username)
-    User.joins(:user_emails).where("user_emails.email = ? OR username = ?", email.downcase, username).first
+    # Force the use of the index on the 'user_emails' table
+    UserEmail.where("lower(email) = ?", email.downcase).first&.user || User.where(username: username).first
   end
 
   def created_category(category)

script/import_scripts/base/lookup_container.rb

@@ -37,7 +37,7 @@ module ImportScripts
 
     # Get the Discourse Group id based on the id of the source group
     def group_id_from_imported_group_id(import_id)
-      @groups[import_id] || @groups[import_id.to_s] || find_group_by_import_id(import_id).try(:id)
+      @groups[import_id] || @groups[import_id.to_s]
     end
 
     # Get the Discourse Group based on the id of the source group
@@ -47,7 +47,7 @@ module ImportScripts
 
     # Get the Discourse User id based on the id of the source user
     def user_id_from_imported_user_id(import_id)
-      @users[import_id] || @users[import_id.to_s] || find_user_by_import_id(import_id).try(:id)
+      @users[import_id] || @users[import_id.to_s]
     end
 
     # Get the Discourse User based on the id of the source user