How a pure-Python jq ended up 40x faster than the C bindings

I spent yesterday building purejq , a pure-Python implementation of jq. I expected it to be the slow-but-portable option. Then I benchmarked it against the jq package on PyPI (the C bindings everyone uses to run jq from Python) and got this, on a 100k-object array, in-process: workload purejq jq PyPI (C bindings) field-access stream 9 ms 368 ms filter + count 55 ms 442 ms map + aggregate 18 ms 444 ms group_by 112 ms 704 ms transform + sort 136 ms 899 ms Pure Python, 7-40x faster than the C extension. That number looked wrong to me too, so before publishing anything I made the benchmark script verify every output against the actual jq binary first ( tools/bench.py --verify ), re-ran everything as median-of-7, and gave the bindings their best-case API. The gap is real. Here's why. The serialization tax The C bindings wrap real jq, and real jq only speaks JSON. So every call does this: your dicts -> JSON text -> C parser -> jq evaluates -> JSON text -> dicts That round trip costs about 350-450 ms for 100k small objects on my machine, before any actual filtering happens. You can see it in the numbers: even a trivial field access pays the same ~400 ms floor as a group_by. purejq skips the trip entirely. It compiles the jq program once into Python closures and walks your dicts and lists directly: import purejq prog = purejq . compile ( " group_by(.team) | map({team: .[0].team, n: length}) " ) prog . first ( data ) # operates on your objects, no serialization The lesson generalizes beyond jq: when you embed a C library that has its own data model, the marshaling boundary is often more expensive than the work. An interpreter written in your language gets to skip the boundary, and that can buy back an order of magnitude. Surprise number two: the CLI beats the jq binary on big files This one I really didn't expect. End to end on a 93 MB file (1M objects), parse + filter + output: workload purejq CLI jq 1.8.1 binary single lookup 0.51 s 1.68 s filter + count 1.08 s 1.96 s grou