I would suspect that most people don't come close to using all the ram in their computer in normal use. Whether they use adblock or not will not have any noticeable affect on the computer performance apart from the blocking of ads.
Not sure I agree with this. I tend to use way too many tabs, and therefore use up my memory. I was visiting my parents recently, and on their Mac they were leaving a lot of programs running (having closed the window but not exiting the program).
I think low-memory situations are the most likely explanation for slowdowns experienced by typical home or office users.
