The computer is slow after cleaning, why?

Question

Iron

I haven’t cleaned my PC for a long time, I took out a fan, cleaned everything thoroughly with a vacuum cleaner (I smeared last year’s thermal paste a little, but I vacuumed it well). I put everything back in, turn on the PC, and it slows down a lot, and after 15 minutes it freezes for a second and reboots. Replaced thermal paste. It does not overload, but it works slowly, what could be the reason for this?