Diff rename detection performance issues

I have a merge commit that includes 2 modified (!) files: hello/foo/stubs/example.php hello/world.php

I want to only get the changes introduced by the merge commit and exclude any changes in /foo/stubs/: git diff -l0 --name-status --find-renames "$sha"^'!' -- ':!*/foo/stubs/*'

Git takes more than 4 minutes to generate this diff, since hello/foo/stubs/example.php is a huge file. When using --no-renames (instead of --find-renames) it's much, much faster. And without the example.php file, the diff takes less than 1 second instead of 4+ minutes.

Funnily enough, when I have a merge commit that contains only that 1 excluded file, it's the same behavior.

1) if there's only a single file in a commit, why does --find-renames
cause a slowdown? There's nothing that could have been renamed in that
case (probably the same for --find-copies)

2) could rename detection be "delayed" to only run/check if there are
actually additions/deletions (and possibly only check those)? If a
commit only contains modifications (unlike in a really, really 0.0001%
edge case) but no additions+deletions it's extremely unlikely that
there's a rename, so detection could be skipped altogether?