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

Very interesting. It is something I've never hit. Or, rather, I know the symptoms, but in my case they are caused not by the age of the installation, but by the memory use and swapping. When I compile something big, it can eat a lot of memory, and even 32Gb or RAM is not enough sometimes, there are lags and sometimes very painful.

It is really interesting because the behavior was persistent for all Linux distributions you tried, so probably there was some program that ate all the memory. Pity you didn't try running top and watching CPU and memory usage. Now I'll think of possible causes to the end of my days without any hope to find a real cause.