A few months ago I added the AdBlock app to my Mozilla browser. Worked fine for a while, but lately I've noticed lots of adds re-appearing on websites, and my computer was slowing down a bit.
Out of curiosity I uninstalled AdBlock. Websites now load much much faster, even with all the ads !
huh ?
Internet search reveals lots of info about AdBlock actually using more processing power in their attempt to block an ad, than the original ad uses. Hence a slower computer.
So, if you have AdBlock, and your computer seems a tad slow, try un-installing AdBlock and see if it makes a speed difference. Certainly did for me.