[nwjsmith]

Harper checks a lot of your boxes and is getting better all the time: https://writewithharper.com/

[jgalt212]

How does Harper compare with LanguageTool. We use a privately hosted version. It's better than nothing, but in practice it's more like a super-charged spell checker.

[hiatus]

On the homepage it has a comparison of figures, presumably indicating response time, though it doesn't speak to its performance in terms of grammatical errors caught:

Harper - 10 ms LanguageTool - 650 ms Grammarly - 4000 ms