It seems like the person was suggesting if all processes on your PC used a client/server model with message passing/RPC instead of the existing API model, the idle cores you speak about, would not be idle.

While you're right that productivity versus performance is a trade-off, and an editor is not necessarily a high performance application, its not clear to me whether future optimizations would reduce the gap, as much as optimizing compilers did vis-a-vis C and assembly.

In any case, that aside, the core guarantee of software stability with LSP remains to be seen.