EDIT: finally managed to do it by tweaking a register. Weird, my filesystem already contains a mix of files with and without 8dot3names. Will monitor this in the future to see if there is an impact on FS performance.
It strikes me Microsoft struggles with significant legacy across much of their product range, that is holding them back. Between filesystems still supporting 8.3 naming, and client links to AD seemingly assuming that clients are on trusted LAN networks (in 2021), I wonder if their backwards compatibility could be their downfall.
In a competitive market where they weren't quite so dominant, it would be interesting to see if a startup could get good enough API compatibility through WINE that they could offer a commercial "windows replacement" distribution, offering remote management via an AD-compatible interface (that can run zero-trust via SSL, the way it should be designed in 2021).
That's looking at large numbers of files and folders, but we all know that overheads like these tend to come about due to inefficiency at the per-file level scaling up.
Also might be worth checking you've got 8.3 filenames disabled and stripped from existing files (https://docs.microsoft.com/en-gb/archive/blogs/josebda/windo...).
Although in general I must admit I find Linux offers better performance due to less "hidden cruft" running - perhaps the issue is background services like Windows defender (and all the other ones that don't immediately leap out) like you suggest.