Super Mario Bros. (Europe) is sped up by 2x

The Super Mario Bros. (World) ROM is completely normal, however the European version is sped up by exactly 2 times. The sound is pitched up by an octave, the timer is two times as fast, Mario walks at what in normal versions is his running speed, etc.

I doubt this is a problem with my emulator (Bizhawk) because every other NES game runs fine, including regional versions of other Mario games.