This list of tests that regressed are mentioned in the comment below.
Preliminary investigation shows that there are 2 suspicious commits: WT-9432 and WT-9310 but I don't see how they can affect btree load time.
Ensure Evergreen builds have debug information
~20% performance regression across all btree load times
Unable to set HAVE_BUILTIN_EXTENSION_ZSTD: Failed to find zstd library