Multithreading for higher speed

This is a bit of a difficult topic.
It seems that recent PCs are equipped with dual or quad cores, and if we make good use of multi-threading, we can easily double or quadruple the processing power.
Especially in the area of image inspection, which requires high-speed processing, it seems to be quite effective.
However... let me tell you a fact that we have faced.
For each additional thread, all threads slow down by 20%."
As the astute among you may have noticed, when this phenomenon occurs, processing capacity does not increase that easily, and on the contrary, we see the phenomenon of "processing capacity decreases as threads are added" at a surprisingly early stage.
It may depend on the contents of the processing, but I think just keeping in mind the fact that "this kind of thing can happen" will make a big difference. I myself was thinking, "I've been hit! I was like, "Oh my God!
Up to the quad. Anything more than that...