Yeah if you're serializing and deserializing data across the JS-WASM boundary (or actually between web workers in general whether they're WASM or not) the data marshaling costs can add up. There is a way of sharing memory across the boundary though without any marshaling: TypedArrays and SharedArrayBuffers. TypedArrays let you transfer ownership of the underlying memory from one worker (or the main thread) to another without any copying. SharedArrayBuffers allow multiple workers to read and write to the same contiguous chunk of memory. The downside is that you lose all the niceties of any JavaScript types and you're basically stuck working with raw bytes.

You still do get some latency from the event loop, because postMessage gets queued as a MacroTask, which is probably on the order of 10μs. But this is the price you have to pay if you want to run some code in a non-blocking way.