To be fair, this is probably ZFS-on-Linux having a bad swap interaction. But I have found that once I get to 95% RAM use, I start hitting the swap fairly frequently and as a side-effect my desktop starts stuttering (usually happens if I kick off an 8-core code compile).

The other culprit I suspect is too many potentially blocking processes Cinnamon's main thread, but I've never figured out a decent way to go after the problem.