We collapse text in large diffs by default, but show files with inline comments (including lint messages). Because this diff has 250 lint messages, most of the files expand, defeating the "this is a large diff, so collapse to speed it up" feature. D9991 is an example of a diff on this install which is larger (affects more files) but loads very quickly because files collapse by default (as it does not have lint messages). A potential workaround for this is to use --nolint when running arc diff to suppress linting.

In theory we could try to relax the "inlines" rule and try to detect when there are too many inlines, or provide more UI options, but I think this case is very unusual (we haven't seen other similar reports) and the additional complexity cost isn't worth improving this edge case.

See also Differential: Large Changes for some discussion about reviewing (and not reviewing) large changes. I'm not sure if the linked revision is really applicable there, but it seems like a lot of it might have been machine-transformed and be unlikely to receive detailed human review.