Yes, I only started thinking about the json parser recently after some benchmarking showed that it was becoming the next limit on performance after we eliminated some even larger bottlenecks. My original idea was to remove it entirely (one of the other devs was opposed), but then third dev stepped up and pointed out that there were much faster parsers we could use, making us us both happy.
And, yes, this is very important to performance because we can expect that most operations nowadays already are, and will continue to be custom_json, because that's the operation which 2nd layer apps will mostly use (well, we'll probably allow for good custom_binary eventually as well, but that's a story for later).