Possibly an extension running that is chewing up resources, if you install apps/games/etc that use facebook or your browser then this might be the culprit. Possibly browser version compatibility, updating your browser is probably not a bad idea regardless. Clearing out cookies/cache/history is relatively easy and might help. Worst case would be a virus or spyware that is using up resources, keep an eye out for other weird stuff happening, one site being slow isn't a huge red flag IMHO. I admit that I am far from an expert, maybe others will be able to offer better advice, but I thought I'd give you this as a starting point.