OMG!

A Fb user said this:

I had the same problem (dead slow, even with different browsers) until I discovered I had multiple active sessions open.

The cure;
1) (i) Go to settings (the cog on the tool bar at the top of the page), (response will be slow until the problem is fixed, so give it time)
(ii) Select Privacy settings,
(iii) Select security,
(iv) Select edit active sessions,
(v) Close all active sessions except one.
2) Un-click the "stay logged on" button at log-in.
3) "Log out" before exiting Facebook each time.
4) Periodically check the number of active sessions.

Facebook has been working fine for me since I did this.