If the answer is to this question is no, the answer to whether or not a new format is necessary is not automatically yes. Certainly for at least one of those formats, a reasonably performant JS implementation could be created.
We assessed what prior work had been done and found these attempts to ultimately be unsatisfactory performance-wise for the breadth of JavaScript clients we would like to reach.
We have also found that none of the common serialization formats perform acceptably in javascript except JSON (and that's huge on the wire).
As an aside, transit seems dramatically faster in v8 than in firefox, at least in the versons of browsers I'm using, despite the fact that JSON.parse and hydrate are faster in firefox. Has it been specially optimised for v8?
We did not specifically optimize of for V8, the optimizations present resulted in a performance win in all browsers. Firefox simply does not deliver the same performance as V8 or JavaScriptCore for this kind of work. Still I think Transit is plenty acceptable under Firefox for many typical JavaScript programs. Hopefully the existence and usage of Transit will encourage Firefox to further improve their JavaScript performance profile.
Look at JSPerf for other serialization formats vs. JSON. They all look pretty much like this. Compare these results to http://jsperf.com/json-vs-transit/2 where in some cases we beat JSON.
Possibly but in many ways it's a lot easier to pick and write a fast implementation using e.g. typed arrays than it is to convince the world to use your special format
