Facebook should be very fast for logging in. Please feel free to try it from this site to let us know if it's that slow here. If so, that would indicate an internet connection or some other problem between your location and Facebook.
The other possibility is that your server is taking a while to process the user's profile information. I'd recommend disabling any Social Profile plugins you have enabled to see if that speeds things up. If so, the process of fetching the user data from your server is slow. That could be due to an overloaded server or requesting a lot of information about the user. Try reducing the amount of data requested to see if that helps.
Let us know how the tests above go and we'll gladly continue to help however we can.
Thanks,
Alex
why FB login so slow?
