Few things to consider:
Slow drives, mostly HDDs, can hugely impact performance (but should not be causing crashes)
Task manager is not a great way to view utilization but it can do the trick most of the time. The important thing to realize is that the 20% metric refers to the all of the cores. If one core is running at 100% but the rest are sitting at 0 you're going to see a low utilization even though that one core/thread. At the Performance > CPU screen change the graph to 'logical processors' rather than 'overall utilization' as it will give you a more accurate picture of what's going on.
There's a chance that you have a hardware problem, but these issues (when as bad as you've described) usually justify a fresh install of Windows.
From what you've said elsewhere it likely is your HDD but it's still good to take a look at your thermals. In HWInfo click the 'Sensors' button. It'll bring up a fairly long list of different readings covering power usage, utilization, temperatures, and more. Scroll down until you see your CPU and Core temperatures. The 12th gen Intel CPUs have a max temperature rating of 100C and will cut power and clock speeds if that ceiling is being hit. If you see any cores bouncing off that number (or close to it) when under load you have a cooling problem. It should usually be much lower than that though. Depending on your ambient temperature, case, and cooling solution it could range anywhere from 25C to 50C at idle. With any minimally decent cooling setup and while under heavy load 80-90C can be considered normal but a good cooler will usually keep you under 80C, sometimes significantly.