Make use of uncached diffs where highlighting is not required

app/helpers/diff_helper.rb

@@ -162,7 +162,7 @@ module DiffHelper
   end
 
   def render_overflow_warning?(diffs_collection)
-    diff_files = diffs_collection.diff_files
+    diff_files = diffs_collection.raw_diff_files
 
     if diff_files.any?(&:too_large?)
       Gitlab::Metrics.add_event(:diffs_overflow_single_file_limits)

app/serializers/diffs_entity.rb

@@ -42,13 +42,13 @@ class DiffsEntity < Grape::Entity
 
   # rubocop: disable CodeReuse/ActiveRecord
   expose :added_lines do |diffs|
-    diffs.diff_files.sum(&:added_lines)
+    diffs.raw_diff_files.sum(&:added_lines)
   end
   # rubocop: enable CodeReuse/ActiveRecord
 
   # rubocop: disable CodeReuse/ActiveRecord
   expose :removed_lines do |diffs|
-    diffs.diff_files.sum(&:removed_lines)
+    diffs.raw_diff_files.sum(&:removed_lines)
   end
   # rubocop: enable CodeReuse/ActiveRecord
 

app/serializers/diffs_metadata_entity.rb

@@ -2,5 +2,5 @@
 
 class DiffsMetadataEntity < DiffsEntity
   unexpose :diff_files
-  expose :diff_files, using: DiffFileMetadataEntity
+  expose :raw_diff_files, as: :diff_files, using: DiffFileMetadataEntity
 end

lib/gitlab/diff/file_collection/base.rb

@@ -8,7 +8,7 @@ module Gitlab
 
         attr_reader :project, :diff_options, :diff_refs, :fallback_diff_refs, :diffable
 
-        delegate :count, :size, :real_size, to: :diff_files
+        delegate :count, :size, :real_size, to: :raw_diff_files
 
         def self.default_options
           ::Commit.max_diff_options.merge(ignore_whitespace_change: false, expanded: false, include_stats: true)

spec/helpers/diff_helper_spec.rb

@@ -259,7 +259,7 @@ describe DiffHelper do
   end
 
   context '#render_overflow_warning?' do
-    let(:diffs_collection) { instance_double(Gitlab::Diff::FileCollection::MergeRequestDiff, diff_files: diff_files) }
+    let(:diffs_collection) { instance_double(Gitlab::Diff::FileCollection::MergeRequestDiff, raw_diff_files: diff_files) }
     let(:diff_files) { Gitlab::Git::DiffCollection.new(files) }
     let(:safe_file) { { too_large: false, diff: '' } }
     let(:large_file) { { too_large: true, diff: '' } }

spec/serializers/diffs_metadata_entity_spec.rb

@@ -36,8 +36,16 @@ describe DiffsMetadataEntity do
 
     describe 'diff_files' do
       it 'returns diff files metadata' do
-        payload =
-          DiffFileMetadataEntity.represent(merge_request_diff.diffs.diff_files).as_json
+        raw_diff_files = merge_request_diff.diffs.raw_diff_files
+
+        expect_next_instance_of(Gitlab::Diff::FileCollection::MergeRequestDiff) do |instance|
+          # Use lightweight version instead. Several methods delegate to it, so putting a 5
+          # calls limit.
+          expect(instance).to receive(:raw_diff_files).at_most(5).times.and_call_original
+          expect(instance).not_to receive(:diff_files)
+        end
+
+        payload = DiffFileMetadataEntity.represent(raw_diff_files).as_json
 
         expect(subject[:diff_files]).to eq(payload)
       end