Question Why does my PC sometimes run significantly slower than usual upon booting?

Mof

Aug 27, 2020
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Upon some boots my computer would run much slower. Everything is laggier, webs load slower, screen freezes for a few frames once so often when typing, and the FPS is also much lower and more inconsistent in games.
So I ran Cinebench on one of these' slow boots', and found that the multicore score is about half of what it should be (3325vs6830) and the single-core is even worse (66vs487), taking over an hour to complete.
It only happens occasionally and a quick reboot can fix it without any issue. But I'm just quite curious why this happens.