I've seen online Word become extremely slow even for small documents with a few editors.
It doesn't help that there are lots of versions of online Word - there's the one that lives in Sharepoint, the one I get to via Outlook, the one in Teams, and (maybe? somehow?) the desktop Word when editing an online file.
In my experience it starts to creak on large documents with multiple editors. I did a project many years ago where the key deliverable was a large report, worked upon by 3-4 contributors over 12 weeks.
That last week was absolute madness. Document got corrupted, and we tried all sorts of things. We ended up importing the corrupt document into Google Docs, which, oddly enough, did a better job of working with a corrupt Word document than MS Word did.