Multithreading is a grand good source of instability and bugs, please don’t.
Multiprocess if you must, but unless you have a language like erlang / elixir multithreading is a bug prone nightmare.
Yeah it sucks that this is missing, but on the other hand you have Emacs server and clients to circumvent this.
The only “performance” issue for me is lsp - but that tends to be at first run on a large framework. And since my “old” laptop (x1c6) is about 1000x faster than the one I started using emacs on in the early 2000s it really isnt an issue.
Less haste, more speed…
It might not be a dealbreaker for me per say, but it’s absolutely something that should be included one day.
Out of curiosity - what are your use-cases where the lack of multi-threading is an issue?
Dealbreaker? No. Major issue? Yes.
Anything over network really needs multithreading. The async hacks are ok, but eventually everything has to sync up again, which is a giant freeze on everything.