Must be something in the Emulator I tried with. This afternoon I installed H.E.R.O. Is Back in my MiSTer and now speed is the same as the PC version. Weird.
I also tried the original H.E.R.O. In NTSC mode on MiSTer and the sounds are faster there, too. It looks like there was never a NTSC/PAL fix for that game. Maybe it would be nice for all NTSC users to have the same experience…