You know what... I did mess with something... I wanted to see what would happen if I changed the display language to Japanese. Normally I can read Japanese but it turned out I didn't have the fonts installed or something, so I got a lot of funny boxes

I got a little bit turned around in the menu system, but luckily it didn't take long to find my way back to English.
But now that you mention it, it might very well have been
right after doing that that the slowness started, because everything was fine at the very beginning... now I have something I can actually test.
Thanks,
Dave