Message passing without shared memory is slow, having to copy data and maybe even serialise/deserialise it in the process. Message passing where you are just passing a pointer is fast.

Web workers are basically the worst case, you have to serialise your data to and from JSON when passing it to and from a worker. It’s not built for performance. There have been many cases where people have tried to improve performance of their apps by offloading work to a web worker but the added cost of serialisation ultimately made it slower than running on the main thread.