Did it always do that, or did it start after an update? Personally, I am not liking this version 11 of Tor. It takes a strange pause before going to the pages I type in or click. Yesterday, all the comments at the Tor Project blog were turned off, so I couldn't read if there were any other complaints. That was strange.
How much RAM do you have, and use at any given time? I mistakenly bought an HP notebook a few years ago that was good for little more than a paperweight. It only had 4 gigs of RAM, and most of its storage capacity was already used up straight out of the box.
You can also check task manager to see what is running, what's hogging memory, what's using the most CPU, etc.
It's possible you accidentally set something in Settings, like a "Keep Tor on top" option or something. I have accidentally changed settings before, mostly when I fall asleep sitting on my keyboard.
You would know these, but do a defrag every month or so or whenever needed. Use CCleaner or something like it to scrub away all the garbage in your temp folders. Check your registry with a utility program to make sure it's healthy, but never delete anything there unless you are certain it won't mess up your system.
I store all saved things downloaded onto an external memory device and scan frequently for viruses. This way none of it is actually residing on my computer.