Not sure how to debug this as it's on a private repo, but both firefox and Chrome are hanging when I try to diff version 12 of a PR against latest. Attaching a firefox trace I captured of the issue (symbols appear to be minified so not sure how clear it'll be to someone what's going on)
Firefox 2026-01-13 17.13 profile.json.gz
Here's the chrome trace:
Trace-20260113T171727.zip