I tested it again as requested (strace with same 'hg diff' call), the results
are as follows

With hg revision b4eaa68dea1b:
475 stat calls, 2 getdent calls

With hg revision dccb83241dd0:
25523 stat calls, 4633 getdent calls

With most current revision 8fa54b9c6c5a:
31084 stat calls, 4644 getdent calls

So there is a significant increase with the version you mentioned, however the
most current version is even more worse.

Even with the best results (475 calls) this is still a lot compared to Git (12).

I hope this has helped, if you need more info please contact me anytime.

Thanks!