I had a laptop that a customer kept complaining about being slow. The problem was, it was only slow in certain places. I looked at the laptop again and again and could find nothing wrong with it. And I never once saw it slow down. I had several ideas, one of which was the outlet power wasn’t clean causing weird problems with the laptop.
It turns out the power adapter was too weak and once removed from the laptop, it began to work well again. The customer figured that one out, not me.
Another customer had a computer that wouldn’t turn on. The power supply and the power cord were both bad. After replacing the two, his computer worked much faster.
If a computer is running slow and no other cause can be found, replace the power supply or power adapter and the power cord and see if it speeds up.