Have been playing with xdebug and Kcachegrind a bit and discovered to my horror that get_username_string() accounted for 27% of a simple viewforum.php pageview. Even worse results on an index page.

Have added some static luv and moved some return statements up. With those changes get_username_string() uses 2% instead of 27%

Currently fairly untested code, it's more like a proof of concept at the moment

Patch has been attached.

Ps. small bug in the patch, one of the last lines should be '$mode == 'profile', not '$mode != 'no_profile')