Universal out-of-the-box support in almost every language. The actual performance impact is subtle - for most core/front-end interactions, the messages are very small (because we put so much effort into minimizing the deltas). It's also the case that there are ridiculously fast JSON implementations out there. We did discover that Swift's is not one of them though. I have a prototype of a faster one using Codable; I'd consider upstreaming it to Swift but just need to find the time.
I'd consider a different serialization format, but it doesn't seem to be on the critical path for either performance or functionality, so I feel there are a lot of other things ahead of it.
To second that the serialization format is unlikely to matter: I'm doing my own editor, and I don't put any effort into minimizing deltas, and retransmit more than the full visible window content on every keypress currently. I expected to have to change that to get it fast enough, but latency just isn't noticeable so I've bumped it down to the bottom of my priority list.
gRPC is not necessarily the magical tool everyone would like to believe. It is mostly developed by Google with a lot of magic in it. For instance, in Python using gRPC is totally incompatible with multiprocessing (meaning: it will crash both processes) and has been for a year (1.4.0...1.8.4), because of all the C logic (and multiple threads) running behind it.
I for one am glad they didn't go towards custom protocols with single complex implementations, towards something that can be assembled easily from standard components.
I'd consider a different serialization format, but it doesn't seem to be on the critical path for either performance or functionality, so I feel there are a lot of other things ahead of it.