Yes, --trace-gc shows about 10 Mark-sweeps per second, each taking around 13 ms (no compacts though as far as I could see). But are those ~15% spent in GC are enough to explain the performance?
You were saying that V8 GC is failing here so I just explained why JSON.parse is especially bad for V8 GC.
Strictly speaking I am not even convinced that GC is bottleneck here. Only profiling can reveal the real bottleneck.
[I tried small experiment: used thirdparty pure-JS JSON parser instead of V8 JSON.parse --- that changed GC profile, but did not affect response time.]
