TEMU, a Game Boy emulator for the ZX Spectrum +2a/+3 capable only of running a Tetris ROM.
As far as CPU goes only 9 instructions had to be implemented via emulation, everything else is executed directly. The rest of the resources are spent emulating graphics and sound to the limited extent that Tetris is using them. Keep in mind that the Spectrum's CPU runs slower than that of the Game Boy. 3.5 Mhz vs 4.19 Mhz.
(Real hardware capture)