Frameskip is set to automatic, so it's possible what you are noticing is missing frames. If you run it from the command line (/opt/mame/mame.sh tumblepo (or whatever the mame name for tumblepop is)), you will see afterwards what the actual speed was. There's also a way to access that information from the menu (one of the volume buttons will open menu).

In SF2 i usually get it during the first fight but not always. I've been getting it in other games too but not as predictably (in GnG it hasn't ever happened). I'm still not sure that it's not a peculiarity with the setup on my N900 because i haven't had complaints, so that's why i ask.