Speed up json.dumps() for small documents

[gaborbernat]

json.dumps() runs through the pure-Python iterencode() wrapper on every call, even though the C accelerator does the actual encoding. That wrapper builds a float-formatting helper closure each time it runs, regardless of whether the document contains any floats and regardless of whether the fast C path (which never uses the helper) is taken. For the small documents that dominate real traffic, that per-call setup is a measurable slice of the work.

Most JSON crossing a service boundary is small: an API response, a single record, a config fragment. Web frameworks, RPC layers and log shippers encode these short payloads constantly, one per request or record, so the fixed per-call cost adds up.

Over 20 small documents, building the float helper only on the slower Python encoding path makes json.dumps() about 8% faster, consistently across float-heavy, mixed and no-float payloads. The win applies to the default build with the C accelerator, since the wrapper that carries this overhead is written in Python.

Linked PRs

• gh-150820: Speed up json.dumps() for small documents #150827