O_o (./23) :

CosmicR (./19) :

O_o (./7) :
doom on a 12mhz cpu ... well, forget it
maybe wolfenstein with agressive optimisations (like on megadrive) but only possible on neogeo cd

There's a version of Doom on the ZX Spectrum - uses a 3.5mhz Z80 as its CPU (Basically a slower version of the Neo Geo's sound processor)

This is NOT a Doom port, this is a remake or demake with extremely degraded graphics and 3D capabilities
it was remade from scratch with ZX spectrum limitation in mind, it's not a straight port of the original source code

Like official Doom/Wolf3D on the SNES/Megadrive and other platform, most of them don't use the original PC code, and for some use the original asset, lots don't.