Ended up re-installing Windows 10 from scratch, and like always before, that solved all the problems! No more slowness or jerky mouse. Everything is faster now and the touchpad mouse is smooth again, and even Photoshop got a lot faster. Glad I did it – even though getting all my programs re-installed and all the Windows settings as I like them took a whole day – ’cause it feels almost like a new laptop again!
It’s so stupid Windows requires doing this about every year or two. I’ve had Linux Mint installed for like five years now and it’s upgraded every time there’s a new version, but it never gets slower or gives me any other problems like Windows does after X number of upgrades.