On 8 threads, Python 3.15 free-threaded ran pure-Python code 3.83 times faster, JSON parsing 4.49 times and sorting 4.25 times, while the standard build stayed at about 1.0 times.
The free-threaded build was 10 to 12 percent slower on a single thread for pure-Python loops and JSON parsing, so it only pays when two or more cores stay busy.
An unlocked shared counter lost 6.8 million of 8 million updates, 85 percent, on the free-threaded build; a thread-local count merged once fixed it in 0.048 seconds against 0.80 seconds for a