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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.
What evidence do you have for this?
Magit
Networking activity is a prime example of something where threading is least likely to be a clearly superior solution. Heavy CPU use is where threading may be the superior choice and a single event-loop thread is not. IO waiting is not.