I expected this to be very slow. Access to Facebook's API is slow. My Facebook app that analyzes mutual likes is very simple and has just one stat, but it could take a second to query data for every 10 friends. The more friends you have the slower it is. You can't use just one query for all your friends either. You have split queries and wait for each before querying for the next batch.

I made something related to that extension, but instead of counting the number of hits, it keeps track of how much time you spend on different websites: http://webtimerapp.com