I have a PHP application running in iFrame mode. I am rendering an <fb:multi-friend-selector condensed="true"> inside of <fb:serverfbml> tags. This is inside a PHP file that calls a function that gets a list of user IDs using $facebook->api_client->friends_get();. The multi-friend selector renders just fine, but, when I leave the friend_get() call uncommented, the page takes between 15-20 seconds to load (confirmed with Firebug)! The goal is to limit the number of users displayed in the selector by building a list of user ids not to display, for use in the friend selector's exclude_ids parameter. And since it's "exclude_ids" and not "include_ids", I can't think of a way of getting around this api call. It seems to me there must be something I can do to make the api call faster, because I've seen friend selectors that load much more quickly.
After over a month of ripping my hair out over this issue, I discovered a fairly feasible workaround. The PHP API calls will work extremely slowly from any AJAX requests you make. This likely has something to do with Facebook parameters being missing, or some other such nonsense.
The workaround works like this: instead of calling the Facebook API function from the PHP file being called via AJAX, make sure you isolate all PHP calls to the Facebook API to the index file loaded when the app is first loaded. Save the returned values into a session variable, and you can now load those values in whatever subsequent AJAX calls you make.