at least in the frontend (without WASM), it depends.

a few months ago i tested https://github.com/mapbox/pbf and while it was faster for deep/complex structs vs an unoptimized/repetative JSON blob, it was much slower at shallow structs and flat arrays of stuff. if you spend a bit of time to encode stuff as flat arrays to avoid mem alloc, JSON parsing wins by a lot since it goes through highly optimized C or assembly, while decoding protobuf in the JS JIT does not.

of course it's not always feasible to make optimized over-the-wire JSON structs if you have a huge/complex API that can return many shapes of complex structs.