That sounds weird. I'm not sure when it comes to MP3 files but at least for FLAC files, if you have sufficient padding, no rewrite should happen at all unless the change you make exceeds the free padding (which can happen when you for example embed an image).
I'd suggest you do this instead. Copying a batch of files from or to the server can happen unattended and changing files locally (at least if you have an SSD) should happen pretty much instantaneously.
While you have the files locally it would also be a good time to ensure that your FLAC files have enough padding so that future tag changes (even on the slow server) will happen quickly and without rewrites.
If you're unsure what padding is, I've tried to explain it here.