I have a 2013 Macbook Air with Mountain Lion and while it had been running flawlessly since the day I bought it, a few days ago it started lagging really, REALLY bad. I've been able to narrow down the problem and now I'm 99% positive Safari is the culprit here. Whenever I open a new tab (even with some light, text-mostly content) the "Web Content (Safari)" process starts to devour the memory like crazy, sometimes going above 100% and almost never going below 60%. Sometimes it goes down after a while but sometimes I have to close the tab completely to make it come back to normal. The pages with videos are even worse in that aspect, the whole system starts lagging as if I had a 8-year old machine on Vista.
I searched through the forums and found that for some, links misplaced in the reading list were the culprit but I don't see anything out of the ordinary videos and articles there. What else may I try?