All evidence to the contrary, computers can really only do one thing at a time. OK, a dual-core or dual-processor machine can do exactly two things at a time, a quad core can do four, and so on. But a single CPU can do exactly only one thing at a time. It just switches between them all really, really quickly.
So, when one program needs all of the CPU's attention, other programs that need the CPU might not get enough time to do their work. That typically results in a very slow system from a user's perspective.
We can use Task Manager to figure out who the culprit is, but I very much prefer the free download
Process Explorer. Download and run it, and you should see something very much like this:
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