UPDATE: LastPass have fixed the issue in their latest beta version! Thus, this problem will disappear in the next big version update. Good job!
If you’re running Firefox (any recent version, including nightlies), as well as LastPass to store your passwords, you might have noticed that Facebook in particular lags a lot when loading new news items.
After some research, the problematic option is “Click Icons in Fields”, under “Notifications”. Disable that, and everything should be ok.
Luckily enough, disabling LastPass altogether is not necessary. Yet. I’ve informed LastPass about it, so hopefully they’ll solve it in future versions.
(My research was to run a profiler on FB and see what is using the whole computation time. It was LastPass.)