It happens with every rom whether they have many different controls to announce (MK2, SF2) or just a couple (Galaga, DK, Pacman). LedBlinky doesn't start speaking until after the game startup screen is done and Mame launches. In RocketLauncher it announces the controls while it loads.
I've tried changing the startup screen display time and that doesn't have any effect, it just waits until the game starts and then starts talking over it.