diff --git a/Gemfile b/Gemfile
index 458c7ca24..898d6802a 100644
--- a/Gemfile
+++ b/Gemfile
@@ -56,6 +56,7 @@ gem 'clockwork'
 gem 'puma-metrics'
 gem 'puma_worker_killer'
 gem "rack-timeout", require: "rack/timeout/base"
+gem "parallel"
 
 group :production do
   gem 'unicorn', :platforms => :ruby
diff --git a/Gemfile.lock b/Gemfile.lock
index 7ba51b71a..bbc88a030 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -561,6 +561,7 @@ DEPENDENCIES
   newrelic_rpm
   nokogiri
   oauth2
+  parallel
   pg
   pry-byebug
   pry-rails
diff --git a/app/logical/bulk_update_request_processor.rb b/app/logical/bulk_update_request_processor.rb
index d91f48c1e..6cf5c9078 100644
--- a/app/logical/bulk_update_request_processor.rb
+++ b/app/logical/bulk_update_request_processor.rb
@@ -137,45 +137,47 @@ class BulkUpdateRequestProcessor
 
   # Process the bulk update request immediately.
   def process!
-    commands.map do |command, *args|
-      case command
-      when :create_alias
-        TagAlias.approve!(antecedent_name: args[0], consequent_name: args[1], approver: approver, forum_topic: forum_topic)
+    CurrentUser.scoped(User.system, "127.0.0.1") do
+      commands.map do |command, *args|
+        case command
+        when :create_alias
+          TagAlias.approve!(antecedent_name: args[0], consequent_name: args[1], approver: approver, forum_topic: forum_topic)
 
-      when :create_implication
-        TagImplication.approve!(antecedent_name: args[0], consequent_name: args[1], approver: approver, forum_topic: forum_topic)
+        when :create_implication
+          TagImplication.approve!(antecedent_name: args[0], consequent_name: args[1], approver: approver, forum_topic: forum_topic)
 
-      when :remove_alias
-        tag_alias = TagAlias.active.find_by!(antecedent_name: args[0], consequent_name: args[1])
-        tag_alias.reject!(User.system)
+        when :remove_alias
+          tag_alias = TagAlias.active.find_by!(antecedent_name: args[0], consequent_name: args[1])
+          tag_alias.reject!(User.system)
 
-      when :remove_implication
-        tag_implication = TagImplication.active.find_by!(antecedent_name: args[0], consequent_name: args[1])
-        tag_implication.reject!(User.system)
+        when :remove_implication
+          tag_implication = TagImplication.active.find_by!(antecedent_name: args[0], consequent_name: args[1])
+          tag_implication.reject!(User.system)
 
-      when :mass_update
-        BulkUpdateRequestProcessor.mass_update(args[0], args[1])
+        when :mass_update
+          BulkUpdateRequestProcessor.mass_update(args[0], args[1])
 
-      when :nuke
-        BulkUpdateRequestProcessor.nuke(args[0])
+        when :nuke
+          BulkUpdateRequestProcessor.nuke(args[0])
 
-      when :rename
-        TagMover.new(args[0], args[1], user: User.system).move!
+        when :rename
+          TagMover.new(args[0], args[1], user: User.system).move!
 
-      when :change_category
-        tag = Tag.find_or_create_by_name(args[0])
-        tag.update!(category: Tag.categories.value_for(args[1]))
+        when :change_category
+          tag = Tag.find_or_create_by_name(args[0])
+          tag.update!(category: Tag.categories.value_for(args[1]))
 
-      else
-        # should never happen
-        raise Error, "Unknown command: #{command}"
+        else
+          # should never happen
+          raise Error, "Unknown command: #{command}"
+        end
       end
-    end
 
-    bulk_update_request.update!(status: "approved")
-  rescue StandardError
-    bulk_update_request.update!(status: "failed")
-    raise
+      bulk_update_request.update!(status: "approved")
+    rescue StandardError
+      bulk_update_request.update!(status: "failed")
+      raise
+    end
   end
 
   # The list of tags in the script. Used for search BURs by tag.
@@ -255,7 +257,7 @@ class BulkUpdateRequestProcessor
     normalized_consequent = PostQueryBuilder.new(consequent).parse_tag_edit
 
     CurrentUser.scoped(user) do
-      Post.anon_tag_match(normalized_antecedent.join(" ")).find_each do |post|
+      Post.anon_tag_match(normalized_antecedent.join(" ")).reorder(nil).parallel_each do |post|
         post.with_lock do
           tags = (post.tag_array - normalized_antecedent + normalized_consequent).join(" ")
           post.update(tag_string: tags)
diff --git a/app/logical/tag_mover.rb b/app/logical/tag_mover.rb
index ae143e8e4..9930de237 100644
--- a/app/logical/tag_mover.rb
+++ b/app/logical/tag_mover.rb
@@ -69,7 +69,7 @@ class TagMover
 
   # Retag the posts from the old tag to the new tag.
   def move_posts!
-    Post.raw_tag_match(old_tag.name).find_each do |post|
+    Post.raw_tag_match(old_tag.name).reorder(nil).parallel_each do |post|
       post.lock!
       post.remove_tag(old_tag.name)
       post.add_tag(new_tag.name)
diff --git a/app/models/application_record.rb b/app/models/application_record.rb
index 07779931e..82850af8f 100644
--- a/app/models/application_record.rb
+++ b/app/models/application_record.rb
@@ -166,6 +166,33 @@ class ApplicationRecord < ActiveRecord::Base
     end
   end
 
+  concerning :ConcurrencyMethods do
+    class_methods do
+      def parallel_each(batch_size: 1000, in_processes: 4, in_threads: nil)
+        # XXX Use threads in testing because processes can't see each other's
+        # database transactions.
+        if Rails.env.test?
+          in_processes = nil
+          in_threads = 2
+        end
+
+        current_user = CurrentUser.user
+        current_ip = CurrentUser.ip_addr
+
+        find_in_batches(batch_size: batch_size, error_on_ignore: true) do |batch|
+          Parallel.each(batch, in_processes: in_processes, in_threads: in_threads) do |record|
+            # XXX In threaded mode, the current user isn't inherited from the
+            # parent thread because the current user is a thread-local
+            # variable. Hence, we have to set it explicitly in the child thread.
+            CurrentUser.scoped(current_user, current_ip) do
+              yield record
+            end
+          end
+        end
+      end
+    end
+  end
+
   def warnings
     @warnings ||= ActiveModel::Errors.new(self)
   end
diff --git a/app/models/tag_implication.rb b/app/models/tag_implication.rb
index 884094a91..d1062e927 100644
--- a/app/models/tag_implication.rb
+++ b/app/models/tag_implication.rb
@@ -146,7 +146,7 @@ class TagImplication < TagRelationship
 
     def update_posts!
       CurrentUser.scoped(User.system) do
-        Post.system_tag_match("#{antecedent_name} -#{consequent_name}").find_each do |post|
+        Post.system_tag_match("#{antecedent_name} -#{consequent_name}").reorder(nil).parallel_each do |post|
           post.lock!
           post.save!
         end
diff --git a/test/unit/application_record.rb b/test/unit/application_record.rb
index 543e43f3e..60355d482 100644
--- a/test/unit/application_record.rb
+++ b/test/unit/application_record.rb
@@ -21,4 +21,28 @@ class ApplicationRecordTest < ActiveSupport::TestCase
       assert_equal(@tags.reverse, Tag.search(updated_at: ">=#{@tags.first.updated_at}"))
     end
   end
+
+  context "ApplicationRecord#parallel_each" do
+    context "in threaded mode" do
+      should "set CurrentUser correctly" do
+        @user1 = create(:user)
+        @user2 = create(:user)
+
+        CurrentUser.scoped(@user1, "1.1.1.1") do
+          Tag.parallel_each do |tag|
+            assert_equal(@user1, CurrentUser.user)
+            assert_equal("1.1.1.1", CurrentUser.ip_addr)
+
+            CurrentUser.scoped(@user2, "2.2.2.2") do
+              assert_equal(@user2, CurrentUser.user)
+              assert_equal("2.2.2.2", CurrentUser.ip_addr)
+            end
+
+            assert_equal(@user1, CurrentUser.user)
+            assert_equal("1.1.1.1", CurrentUser.ip_addr)
+          end
+        end
+      end
+    end
+  end
 end
diff --git a/test/unit/bulk_update_request_test.rb b/test/unit/bulk_update_request_test.rb
index d5f9c0e3c..0930e0876 100644
--- a/test/unit/bulk_update_request_test.rb
+++ b/test/unit/bulk_update_request_test.rb
@@ -266,6 +266,7 @@ class BulkUpdateRequestTest < ActiveSupport::TestCase
         should "update the tags" do
           assert_equal("bar", @post.reload.tag_string)
           assert_equal("approved", @bur.reload.status)
+          assert_equal(User.system, @post.versions.last.updater)
         end
 
         should "be case-sensitive" do
@@ -288,6 +289,7 @@ class BulkUpdateRequestTest < ActiveSupport::TestCase
         should "rename the tags" do
           assert_equal("bar blah", @post.reload.tag_string)
           assert_equal("approved", @bur.reload.status)
+          assert_equal(User.system, @post.versions.last.updater)
         end
 
         should "move the tag's artist entry and wiki page" do
@@ -351,6 +353,7 @@ class BulkUpdateRequestTest < ActiveSupport::TestCase
 
           assert_equal("foo", @post.reload.tag_string)
           assert_equal("approved", @bur.reload.status)
+          assert_equal(User.system, @post.versions.last.updater)
         end
 
         should "remove implications" do
@@ -370,6 +373,7 @@ class BulkUpdateRequestTest < ActiveSupport::TestCase
 
           assert_equal([], @pool.post_ids)
           assert_equal("approved", @bur.reload.status)
+          assert_equal(User.system, @pool.versions.last.updater)
         end
       end
 
@@ -514,6 +518,10 @@ class BulkUpdateRequestTest < ActiveSupport::TestCase
         assert_equal("approved", @bur.reload.status)
       end
 
+      should "update the post as DanbooruBot" do
+        assert_equal(User.system, @post.versions.last.updater)
+      end
+
       should "set the BUR as failed if there is an unexpected error during processing" do
         @bur = create(:bulk_update_request, script: "alias one -> two")
         TagAlias.any_instance.stubs(:process!).raises(RuntimeError.new("oh no"))
diff --git a/test/unit/tag_alias_test.rb b/test/unit/tag_alias_test.rb
index 8f614be00..e4b369231 100644
--- a/test/unit/tag_alias_test.rb
+++ b/test/unit/tag_alias_test.rb
@@ -142,6 +142,8 @@ class TagAliasTest < ActiveSupport::TestCase
 
       assert_equal("bbb ccc", post1.reload.tag_string)
       assert_equal("ccc ddd", post2.reload.tag_string)
+      assert_equal(User.system, post1.versions.last.updater)
+      assert_equal(CurrentUser.user, post2.versions.last.updater)
     end
 
     should "not validate for transitive relations" do
diff --git a/test/unit/tag_implication_test.rb b/test/unit/tag_implication_test.rb
index f0f8d3625..2b33b6207 100644
--- a/test/unit/tag_implication_test.rb
+++ b/test/unit/tag_implication_test.rb
@@ -124,6 +124,8 @@ class TagImplicationTest < ActiveSupport::TestCase
 
       assert_equal("sword weapon", p1.reload.tag_string)
       assert_equal("sword weapon", p2.reload.tag_string)
+      assert_equal(User.system, p1.versions.last.updater)
+      assert_equal(CurrentUser.user, p2.versions.last.updater)
     end
 
     context "when calculating implied tags" do