Judging from that link you sent, my guess it has something to do with the non-ascii filenames. You’ve got a lot of files there with (russian?) names.
I also see a few dots in those file names, which have been known to cause issues in the past.
Since you’re a programmer, you could try duplicating the vault, use a Python script or something else to rename every file (and its links) to something using ABC123 ascii characters, and then test again to see if you get the same issue.
If it’s not that, it could be having trouble with the file encoding. I imagine it has to encode the file using a different format the moment different characters are used.
If the file encoding has to account for significantly more characters, maybe that would be enough to lag the application with ~10k notes.