Yes, I was using a wireless keyboard with my Mac.
ah! that could be the crux. As this is a Game Boy Color game in the first place, the emulator build into this web page seems to have problems with some (not all) wireless keyboards. If you can, give it a try with the build in mac keyboard (if you have a macbook) or on a handheld device that can play .gbc games :) That should make the control feel waaaay better!
Sorry for the inconveniences. And thanks for the feedback!