JS/WASM calls are fast in V8, and still seem to be improved from time to time (e.g. see: https://v8.dev/blog/v8-release-90#webassembly), not sure about any large data optimizations (TBH I'm not sure what this is about though, because usually one would use JS slices into the WASM heap to avoid redundant copying)
That works if the data is already in the Wasm linear memory and you need to access it from JS. If you have strings (or whatever) in JS, you need to copy them into the linear memory for the Wasm module to use.