Never Mind, but thanks for reading.

Posting my request got me thinking. I booted into my backup volume and deleted a bunch of preferences that I either clearly didn't need, or looked like they might be relevant. E.g. Finder preferences, System Preferences.

When I booted back into my main 9.1 volume performance was back to being snappy.

I suspect that something like Finder Preferences was corrupted and every time the system accessed it things got slowed wayyyyy down.

At least I hope so. We'll see if it comes back.