Running El Capitan on my imac when browsing facebook on safari it is very slow and laggy. E.G. this evening I types a reply and it took over a minute to put the message on screen! I type looking at keyboard not screen. I finished typing to note nothing on screen, then one letter at a time appeared very slowly. scrolling is also laggy as can be. It is better but still little laggy in firefox. not tried chrome. Was not this bad few weeks ago. Googling it, it seems a common issue with safari.