Summary

tests/test_interactiveshell.py::test_stream_performance (added in #14941 alongside the fix for #14937) asserts a 10-second wall-clock budget for printing 250k lines:

def test_stream_performance(capsys) -> None:
    """It should be fast to execute."""
    src = "for i in range(250_000): print(i)"
    start = time.perf_counter()
    ip.run_cell(src)
    end = time.perf_counter()
    capsys.readouterr()
    assert end - start < 10

This passes on developer machines (~6.7s on an M1-class laptop) but flakes hard on shared CI / distro build hosts where 250k prints under load takes 25–30 seconds. It's currently a hard build failure for ipython 9.5.0 in nixpkgs:

assert 29.643374418010353 < 10

The original intent — defending against the O(n²) regression in #14937 — is sound. The wall-clock proxy is the problem: it conflates machine speed with algorithmic complexity. Anyone reproducing the bug on a Raspberry Pi or a busy CI runner sees the test fail without there being any actual regression.

Fix

Replace the wall-clock budget with a scaling ratio test: run the same workload at two input sizes (10k and 100k) and assert that 10× the work takes less than 25× the time.

For O(n) behaviour, the ratio should be ~10. For an O(n²) regression, the ratio is ~100. A threshold of 25 splits the difference cleanly and is independent of host speed.

Empirical numbers

Measured on this branch:

Threshold of 25 sits comfortably between worst-case post-fix noise (11.7) and the regression signal (39.9).

Validation

• 5/5 sequential runs pass on idle host (post-fix code)
• 5/5 sequential runs pass under 8 × yes > /dev/null CPU saturation
• 3/3 sequential runs fail when the fix in IPython/core/interactiveshell.py is locally reverted (bundle["stream"] = "" + += data)
• Total test runtime drops from ~6.7s to ~3s as a side effect (smaller sample sizes)

Notes

• Renamed test_stream_performance → test_stream_scales_linearly because that more accurately describes the asserted property. If you'd prefer to keep the old name for grep/release-notes continuity, happy to rename in a follow-up.
• Docstring includes the empirical numbers and a pointer back to Printing many lines of output is ~50x slower in ipython v9.1.0 and above compared to v9.0.2 and below #14937 / Fix performance of streaming long text #14941 so the next person to touch this knows what threshold-tweak risks they're taking.

Test plan

• Local repro of the failure (regression reverted) — fails consistently
• Local validation of the fix (regression restored) — passes consistently, idle and under load
• CI matrix on this PR