[deadstrobe5]

Summary

escape_string() walked the value one character at a time and sliced s[i:] at every position as soon as the value held anything to escape, so a value with a quote cost time quadratic in its length. It now builds one regex from the sequences actually present (longest first, so """ still matches whole) and replaces them in a single re.sub. A value with nothing to escape returns early. StringType.escaped_sequences and invalid_sequences now return sets built once at import instead of rebuilding them on every call, and SingleKey checks bare keys against a module frozenset instead of concatenating the allowed characters for every character of the key.

Every string value and every new key goes through this path when a document is built from Python data, which is how Poetry writes poetry.lock and how most tools call tomlkit.dumps(dict). Same-process A/B on CPython 3.14, 7 alternating rounds, median:

Workload	Before	After	Speedup
Build + dumps() of a 760 KB poetry.lock (255 packages)	1651 ms	991 ms	1.67x
Build + dumps() of a 198 KB Cargo.lock	616 ms	317 ms	1.94x
Build + dumps() of a small pyproject.toml	6.93 ms	5.05 ms	1.37x
tomlkit.string('a"b')	29.3 us	10.6 us	2.77x
dumps() of one 20 KB value with quotes	44.4 ms	0.73 ms	61x

Output is unchanged. A differential fuzz (about 170k cases: random strings with quotes, backslashes, CR/LF, control characters, """ and ''', for all four string types, keys, String inputs, concatenation, pickling and dumps()) gave identical results. The function always returns a plain str, also for str subclasses such as String, as before. The new test covers repeated quotes and every escape kind in one value.

Duplicate Check

• I searched the open PRs and found none that already fixes this issue.
• Related open PRs (if any, and why this PR is still needed): Re-escape strings concatenated across quote styles #653 also touches String.__add__ around escape_string(), but only its call site, not the escaping itself.

Agent Drafting Metadata

• Agent: Claude Code
• Model: Claude Opus 5.5
• Notes: the agent profiled the write path, wrote the change and the test, and ran the differential fuzz and the benchmarks above. Human attestation is pending.

[human-attestation]

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