Considering it's a RISC emulator running on top of an x86 emulator running in JS, the slowness is perfectly understandable

But when running the 128bit demo I get a funny value for sqrt(2) in fp64 mode: 1.728102266788482

Well, it's definitely fast enough not to notice excessive latency in the terminal and to run simple applications, but the instructions are interpreted so expect a serious slowdown compared to recompiling or dynamically translating the code.

On the other hand, this makes the code portable and indeed the 32-bit and 64-bit targets work pretty well even on my ARM AArch32 machine.