Scaling regression in `Tools/ftscalingbench/ftscalingbench.py` generator

[colesbury]

Bug report

I'm having trouble pinpointing the commit, but there seems to be a substantial regression:

Machine 1 (Intel(R) Core(TM) Ultra 7 155H)

~/cpython$ uv run -p 3.14.2t python Tools/ftscalingbench/ftscalingbench.py  -t 10 --scale 10000 generator --parallel-only
Running benchmarks with 10 threads
generator          407.3 ms
~/cpython$ uv run -p 3.15.0a3t python Tools/ftscalingbench/ftscalingbench.py  -t 10 --scale 10000 generator --parallel-only
Running benchmarks with 10 threads
generator          587.8 ms

Machine 2 (Intel(R) Xeon(R) Gold 6230 CPU)

~/cpython$ uv run -p 3.14.2t python Tools/ftscalingbench/ftscalingbench.py  -t 10 --scale 10000 generator --parallel-only
Running benchmarks with 10 threads
generator          608.5 ms
~/cpython$ uv run -p 3.15.0a3t python Tools/ftscalingbench/ftscalingbench.py  -t 10 --scale 10000 generator --parallel-only
Running benchmarks with 10 threads
generator          1328.7 ms