But depending on teh cartridge you are using its only active in a specific screen.
BLL: Always, speed selectable
T-TRIS: Main menu only, speed fixed to 9600
Lynx Reloaded: Main menu only, speed selectable
Alpine Games: Read manual
Championship Rally: Read manual
PATW: Loader menu, speed selectable
most homebrews where the developer forgot to disable the loader: always, speed fixed to 9600