I cannot thank you enough. That was the one thing I've wanted with certainty for so long, yet never thought of asking.
Just did one test. Seems ok on initial save. Then I tried restructure to Sigil Norm -- I think that's what did it --, all dates were changed. In such a case, I'd think images, mimetype, CSS, and maybe other should be unchanged if unmodified.
Concerning performance, for this purpose, maybe there are faster hashes? A quick look, there are many SO questions about such; and at least a few reports that the fastest seems to be xxHash, or possibly choose one of the faster ones that perhaps has more of a history (MurmurHash or SpookyHash?). Of links I found this may be helpful:
Non-crypto hashes in C++: Farm, City, xxHash, MUM, Spooky 2, Murmur, Metro and t1ha0
https://asecuritysite.com/encryption...e%20lazy%20dog
Of how much of speed difference there is on open and save, mine may not be a good indicator of relative performance compared to some average computer, but I think I can either thru the command-line or AppleScript run some tests and report back on time differences. I have some fairly large EPUBs, a few with thousands of images. That might be helpful.