Improve diff_files endpoint performance

app/assets/javascripts/pages/projects/commit/show/index.js

@@ -33,7 +33,7 @@ if (filesContainer.length) {
   axios
     .get(batchPath)
     .then(({ data }) => {
-      filesContainer.html($(data.html));
+      filesContainer.html($(data));
       syntaxHighlight(filesContainer);
       handleLocationHash();
       new Diff();

app/controllers/projects/commit_controller.rb

@@ -49,7 +49,7 @@ class Projects::CommitController < Projects::ApplicationController
   end
 
   def diff_files
-    render json: { html: view_to_html_string('projects/commit/diff_files', diffs: @diffs, environment: @environment) }
+    render template: 'projects/commit/diff_files', layout: false, locals: { diffs: @diffs, environment: @environment }
   end
 
   # rubocop: disable CodeReuse/ActiveRecord

changelogs/unreleased/326316-improve-async-diff-rendering.yml

+---
+title: Improve diff_files endpoint performance
+merge_request: 59489
+author:
+type: performance

lib/gitlab/diff/highlight.rb

@@ -3,6 +3,8 @@
 module Gitlab
   module Diff
     class Highlight
+      PREFIX_REGEXP = /\A(.)/.freeze
+
       attr_reader :diff_file, :diff_lines, :repository, :project
 
       delegate :old_path, :new_path, :old_sha, :new_sha, to: :diff_file, prefix: :diff
@@ -97,12 +99,12 @@ module Gitlab
         rich_line = syntax_highlighter(diff_line).highlight(
           diff_line.text(prefix: false),
           context: { line_number: diff_line.line }
-        )&.html_safe
+        )
 
         # Only update text if line is found. This will prevent
         # issues with submodules given the line only exists in diff content.
         if rich_line
-          line_prefix = diff_line.text =~ /\A(.)/ ? Regexp.last_match(1) : ' '
+          line_prefix = diff_line.text =~ PREFIX_REGEXP ? Regexp.last_match(1) : ' '
           rich_line.prepend(line_prefix).concat("\n")
         end
       end
@@ -131,7 +133,7 @@ module Gitlab
         # Only update text if line is found. This will prevent
         # issues with submodules given the line only exists in diff content.
         if rich_line
-          line_prefix = diff_line.text =~ /\A(.)/ ? Regexp.last_match(1) : ' '
+          line_prefix = diff_line.text =~ PREFIX_REGEXP ? Regexp.last_match(1) : ' '
           "#{line_prefix}#{rich_line}".html_safe
         end
       end