PERF: Only delete 100 inactive users at once

app/models/user.rb

@@ -728,6 +728,7 @@ class User < ActiveRecord::Base
                      .joins('INNER JOIN user_stats AS us ON us.user_id = users.id')
                      .where("created_at < ?", SiteSetting.purge_inactive_users_grace_period_days.days.ago)
                      .where('us.post_count = 0')
+                     .limit(100)
 
     destroyer = UserDestroyer.new(Discourse.system_user)
     to_destroy.each do |u|