My task in question was a number crunching task, basically doing multiply-and-add for 336-bit integers. I wrote a JS version, and a C version compiled into WASM by using Zig. You'd think that WebAssembly would trounce JS here, but it actually didn't.

The JS code had been written carefully to avoid allocations, and also avoiding the built-in JavaScript BigInt. I rolled my own BigInt instead using an array of numbers. Each number, despite being a double, was basically a 48-bit integer. Long multiplication requires splitting a 48-bit integer into two 24-bit integers so an intermediate multiplication result will fit in 48 bits.

The C version used 32x32=64-bit integer math. (Would have been nice if WASM had supported 64x64=128-bit multiplication)

Even with the overhead of using doubles instead of integers, the JavaScript and C versions ran at nearly the same speed. I think the C version was slightly faster, but not significantly. The C version took a lot longer to load, as it had to instantiate a Webassembly object, and had to run glue code to copy things in and out of Webassembly memory.