Add speed control

I don't have the same reaction as I did in the 80s, but I want to play through the games I couldn't play through as a kid (due to difficulty or lack of time). It seems to me that slowing down the game speed ("emulation speed") would be the only way to do that. Discrete values ​​of 40, 50, 60, 70, 80 and 90 would be sufficient.