I started using adblock when a forum I used to frequent all the time had so many dynamic (flash) ads that my computer would come down to a crawl. The fan would run and it would get so hot I could not leave it on my lap! They really abused the computing power of every visitor that way. It is one thing to show ads, it is another when it kills the performance of the computer viewing it.
It doesn't sound like adverts from your description.
There are websites that nefariously tap into users'computers to do blockchain mining for them ... thus adding to their computing load, and causing the system to go into full load mode. I have encountered it once before and it was pretty strange until I came across such an article about it.
Adverts should not be able to slow down a computer signficantly, perhaps add to the interweb traffic and slow downloading speeds, that's about it.