> Six weeks in, things changed. The Linux installations started to degrade — subtle at first, then undeniable. Random slowdowns. Browser links that wouldn't register for 10 or 15 seconds.

This is something that I would expect with Windows over time (not in a few weeks though) but has never happened to me with Linux. I have run less stable rolling release distros, and I have done multiple major version upgrades on the same machine over time over many machines.

its sounds to me as though you are doing some unusual things - few people use Compiz now because DEs like KDE provide those through their default Window managers.