Google Docs might be able to directly edit .docx files now, but that is pretty new feature, and it doesn't surprise me that it's slow. It's a proprietary format owned by Microsoft. The argument that software ought to be efficient is valid, but the example of editing a non-native format seems unfair. Any time you add emulation layers you should expect things to slow down.
Did you consider trying Microsoft's own browser-based Word editor? It's free too. And .docx is it's native format.
Or, consider doing a conversion to Google Docs native format first (you'll lose some formatting though, possibly a lot of it).
.docx (office open xml) is an open published ECMA standard[0], and lots of other apps include support[1], including google which added editing without conversion in 2019[2]
Did you consider trying Microsoft's own browser-based Word editor? It's free too. And .docx is it's native format.
Or, consider doing a conversion to Google Docs native format first (you'll lose some formatting though, possibly a lot of it).