OK, I have created a bugfix branch bug-4af5ca1921 which modifies
XCopyArea in the case when source and destination are both pixmaps.
That is the situation when TkImgPhotoInstanceSetSize calls it.  It
is simpler because rescaling, for example to account for Retina displays,
is never needed.

With luck this will make your timing comparison better.  Please test
it and report the results, since I am not able to test this myself.
Thanks.