Computer randomly slowing down

Yesterday, at one point, my Internet connection suddenly became extremely slow, to the point where I was unable to load any pages in Chrome. A few minutes later, the issue resolved itself. I saw in Task Manager that System was using a significant percentage of the CPU during that time, while the memory and disk usage seemed normal. Today, the same thing happened again, but this time I noticed Task Manager kept visually freezing, and hung when I tried to close it. I had never had anything like this happen until yesterday (I've had this computer for 10 months now). Why is this happening?

Specs:
Windows 10 1803 build 17134.765 (64-bit)
Intel(R) Core(TM) i7-7500U CPU @ 2.70 GHz 2.90 GHz

I've noticed some comments about recent problems with Chrome, so try another browser and keep Chrome closed.

It's also possible there was some big download or upload happening yesterday in the background. Perhaps the same today, or some other heavy maintenance task like scanning or indexing—the 2 days in a row might be coincidence.