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