Python 3.15's JIT Compiler Is Back on Track — 11% Faster on ARM
After a rocky start where the CPython JIT was often slower than the interpreter in Python 3.13 and 3.14, the project has hit its performance goals over a year early. The 3.15 alpha JIT delivers 11-12% speedups on macOS AArch64 and 5-6% on x86_64 Linux compared to the standard interpreter.
The Comeback Story
Ken Jin, a volunteer contributor, described the journey as anything but a heroic tale. The original CPython JIT in 3.13/3.14 showed practically no speedups, and the Faster CPython team lost its main sponsor in 2025 — casting doubt on the JIT's future.
The turnaround came through community stewardship and a deliberate strategy of breaking complex problems into manageable parts. Instead of massive architectural overhauls, the team opened "mega-issues" that split optimization work into simple, actionable tasks like "try optimizing a single instruction in the JIT."
What Changed
- Region selector improvements — better identification of hot code paths
- Middle-end optimizations — 4 active recurrent contributors (up from 2)
- Detailed contributor instructions — very detailed, immediately actionable guidance that attracted 11 contributors
- Bus factor reduction — goal of 2+ active maintainers in each JIT stage (frontend, middle-end, backend)
Performance Range
The geometric mean numbers (11-12% ARM, 5-6% x86_64) mask a wide range: from a 20% slowdown to over 100% speedup depending on the workload. These are preliminary numbers from the alpha release.
Free-Threading
Free-threading support (no-GIL) is not yet implemented but is targeted for 3.15/3.16, which would make the JIT performance gains available to multi-threaded Python code.
Key Contributors
Savannah Ostrowski, Mark Shannon, Diego Russo, Brandt Bucher, Ken Jin, Hai Zhu, Zheaoli, Tomas Roun, Reiden Ong, Donghee Na, and more — many of whom are volunteers stepping up after the sponsor withdrawal.
Source: Ken Jin's Blog | Benchmarks