Grammarly is indeed a 3rd-party addin for Word and any performance issues you have with that would have to be taken up with its makers. That said, you might try:
a) repairing the Office installation (just to eliminate any potential issues there); and
b) uninstalling, then reinstalling, the Grammarly addin.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]