[metaphorm]

depends on what you're doing.

for the typical AJAX call for some rows of data selected from a datastore and JSON encoded, then no the JSON encoding is not the bottleneck, the network latency and database io time dominate the time it takes to JSON encode the data.

however, consider an alternative kind of task that might, for example, produce a big JSON dump of thousands of records. this is fairly typical of a data export of some kind. the network and database time for this request is the same as for the smaller one, but now instead of JSON encoding 50 records you're encoding 50000 records. it can start to add up. a poorly optimized JSON library will add multiple full seconds to your response time here.