NFS is almost always way too slow to use for your cache. If that's not THE problem, it's certainly A problem.
If in doubt, I would use Redis. But any dedicated cache server will be better than what you have now. Moodle doesn't place a hugely high demand. We run it on the NFS server on a very large site and it runs fine. If you have a spare machine or some VM space you could go that way.
