I think 20% is a pessimistic number, I've seen cases of over 50%. A friends computer (quite decent, i5 8gb) was slow to the point of not being usable at all, op

I think 20% is a pessimistic number, I've seen cases of over 50%. A friends computer (quite decent, i5 8gb) was slow to the point of not being usable at all, opening word would take like 30 sec of disk thrashing. He was on the verge of throwing it out when I took a look at it and uninstalled his antivirus, it became like brand new again.