The big things are indeed being outsourced to separate processes. LSP handles a lot, I also see some attempts at moving syntax highlighting to tree-sitter[0]. But all such work means a good chunk of code is running inside a foreign process, whose behavior cannot be easily inspected or modified at runtime.

But there are a lot of various smaller things in Emacs and elisp module ecosystem that would benefit from being able to run off the main thread. Emacs performance is fighting death by a thousand cuts, with almost everything being tied to the main thread (despite async features being used to pretend it's not).

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[0] - https://tree-sitter.github.io/tree-sitter/