Earlier this year, I tested the experimental JIT compiler included with Python 3.14 (article link at the end). Two of my three workloads became noticeably faster. A Mandelbrot calculation improved by about 25%, a batch of Levenshtein distance calculations by 27%, and a Dijkstra graph search didn’t improve at all.

That was a promising start, although the JIT in 3.14 was still fairly limited. Python 3.15 has since substantially rebuilt it. Rather than repeat the theory from my earlier article, I wanted to ask a more useful question: what happens if I run exactly the same code on both versions?

I tested four configurations:

Python 3.14 with the JIT disabled

Python 3.14 with the JIT enabled

Python 3.15 with the JIT disabled

Python 3.15 with the JIT enabled

This is important because a simple 3.14-versus-3.15 comparison can be misleading. Python 3.15 contains interpreter improvements outside the JIT, including a change to the interpreter used by official 64-bit Windows builds.…