Speed menu removes all BD-J code from the disc and replaces the menu with, simpler, non-Java menus. BD-J is used by most discs to provide fancy menus, navigation control, a custom progress bar etc. but it is notoriously buggy. On more than one occasion, new discs, with slightly different BD-J code, have caused problems with players that have needed new software/firmware before those discs could be played. This is currently true in TMT for the latest Apocalypse Now and Planet of the Apes (original) blu-rays. Also, BD-J can cause performance problems - Fox/Universal discs released since about Sept cause 100% CPU use on one core due to BD-J code, which can cause stutter.
So speed menus are there to:
- let you play discs on players that might not be able to otherwise play them
- solve performance issues that may be caused by BD-J
Oh, and, as the name suggests it also let's the discs load much quicker, missing all warnings, trailers, infomercials etc.