AdBlock shouldn't noticeably slow your browsing. There will be a bit of a delay when you first open your browser because AdBlock is fetching all the filter lists you've subscribed to. After that, pages generally load much faster because you're not downloading all the ads they would normally contain.
There are a couple of ways to minimize the time it takes to fetch your filter lists on startup. First, don't subscribe to more than you need. Second, try not to have a bunch of tabs open automatically every time you launch your browser. If you do, then after AdBlock downloads all those filter rules, it immediately begins applying them to your open tabs to block any ads they may contain.
There are many reasons even a fairly new computer can start to slow down. Add age to that mix and it gets a lot harder to pinpoint any single culprit. We found several articles that suggest ways to speed up an older computer. Here are the ones that look the most comprehensive:
These are all reputable sources. Any software they suggest is probably safe to use. We know that Malwarebytes Anti-Malware and Ccleaner are, at least. Be sure to check the user reviews for any other software the authors recommend before you install it, just to be safe!