As the author says in sibling comment, impersonating a processor is much easier than making an accurate reproduction of period video and sound hardware.
This depends entirely on hardware. You can run good emulation of 8 bit computers (spectrum, c64, cpc) at ~hundreds to thousands the original speed using emulators written in C. Using emulator that barely reaches x10 factor as a performance benchmark for webassembly is questionable.
That's a pretty bold claim considering that (for instance) MAME's CPC emulation eats 25% of one CPU core (checked with vsync enabled). MAME might not be the fastest because they do a lot of the inter-chip-communication via function pointers, but they're also not idling around and the CPC emulation is quite accurate AFAIK. If you can point me to an emulator that's hundreds or thousands of times faster I'd really like to take a look, might steal some ideas ;)