icloud slowing down computer

for several years now all my imacs have been very very slow. I can barely open the browser and using software like photoshop or illustrator causes it to crash.

I've had a large number of video files that I've deleted over the years and i suspect that might be the problem. After i've deleted them I can still search for them in finder and many of them are still there.

but it doesn't matter if I were to wipe the hardrive and start again or buy a new imac it's still very very slow... my icloud is always showing me a message saying that it's full but when I signin to icloud there are hardly any files in it at all.

i don't know what to do to fix this?