Comment by ndiddy

They've been rewriting more and more of the site in React as time's gone on, and all new development has been in React. This means we get fun stuff like diffs being slow to load and failing to load at all when they're too large, the browser back button being unreliable, the code viewer being laggy for viewing large files, etc. Generally they don't seem to care about the site becoming as slow and janky as Gitlab.