[deadstrobe5]

Description

When a processor takes one line off a block and returns the rest, every block processor tests the rest again. HashHeaderProcessor.test and AdmonitionProcessor.test run a regular expression starting with (?:^|\n) over the whole remainder each time, which is slow because the regex has no literal prefix to search for. Tight headers, tight changelogs and Setext headers with no blank lines between them pay this once per line.

This checks for a line starting with # (or for !!!) with a plain substring test before running the regex. The regex needs that text to match, so the result is the same. SetextHeaderProcessor.run also splits only the first two lines instead of splitting the whole block and joining it back.

Median of 5 alternating runs in one process, CPython 3.14, extensions toc, tables, fenced_code, admonition, attr_list, footnotes, def_list, abbr and md_in_html:

Document	Before	After
2,000 headers with no blank lines	2436 ms	913 ms
Tight changelog, 500 entries	327 ms	154 ms
1,000 Setext headers with no blank lines	976 ms	251 ms
2 KB page	1.98 ms	1.90 ms

The old and new code give the same HTML on 30,000 generated documents mixing headers, admonitions, tables, lists and other blocks.

AI Assistance Disclosure

• No AI tools were used in preparing this PR.

• AI tools were used and I have disclosed which ones, and have fully reviewed and verified their output.

  Claude Code (Claude Opus 5.5) was used for everything ticked below. The review attestation in the box above is left for the human submitter.

  AI tools were use to assist with the following:

  • Code
  • Code Comments
  • Documentation
  • Changelog Entry
  • Commit Message
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  • Other: ______________

Checklist

• This PR follows the contribution guidelines.
• The code follows the Code Style Guide.
• The commit message follows the Commit Message Style Guide.
• I have added or updated relevant docs, including release notes if applicable which follow the Documentation Style Guide.
• I have added or updated relevant tests.
• I have not requested, and will not request, an automated AI review for this PR.

[waylan]

Thanks for the work on this.