You vastly overestimate the cost of copying 50k elements. On low end mobile you'll get at least 1GB/sec and on desktop you can get easily 10GB/sec. Now sorting at these speeds is a whole other story.

The real thing to consider is if the latency of the round trip messaging is less than just doing the work in place. With something like web workers you're going to have to hit the event loops when messaging back and forth which aren't always empty.