perf: improve Headers class performance

[anonrig]

This relates to...

N/A

Rationale

Headers is on the fetch / Request / Response hot path. The current implementation pays for WebIDL conversion, token validation, and toLowerCase() as separate passes, and stores every entry as a { name, value } object.

Changes

• Combine ByteString conversion, HTTP token validation, and lowercasing into a single pass.
• Fast-path common lowercase names (content-type, accept, authorization, …) on get / has / delete.
• Store values as strings in the internal Map instead of { name, value } objects. Original casing is kept in a side table only when it differs from the lowercase name.
• Copy an existing Headers instance by cloning the list instead of re-validating every entry.
• Keep the existing fetch spec comments next to the new hot-path code. No changes outside lib/web/fetch/headers.js.

No intended behavioral change. Guard handling, set-cookie / getSetCookie(), iteration order, and original-name casing on the wire are preserved.

Features

N/A

Bug Fixes

N/A

Breaking Changes and Deprecations

N/A

Benchmark

Measured with the source Headers class (Node v22.14.0, 200k iterations, paired old vs new):

Operation	Before	After	Change
get (custom name)	18.4M ops/s	77.9M ops/s	4.2x
get (content-type)	17.6M ops/s	56.2M ops/s	3.2x
has	16.7M ops/s	80.7M ops/s	4.8x
set	15.4M ops/s	25.2M ops/s	+64%
new Headers(headers)	1.12M ops/s	1.72M ops/s	+54%
new Headers()	23.7M ops/s	23.3M ops/s	~same
append	4.36M ops/s	4.33M ops/s	~same
iterate	5.05M ops/s	5.15M ops/s	~same

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented
• Review ready
• In review
• Merge ready

Tested locally:

node --test test/fetch/headers.js test/fetch/headers-inspect-custom.js test/fetch/headerslist-sortedarray.js test/fetch/headers-case.js

All 70 tests passed, including the 4000-iteration toSortedArray fast/slow paths and the fetch cloning cases.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 97.64151% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.46%. Comparing base (f387057) to head (30f9119).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/web/fetch/headers.js	97.64%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5699      +/-   ##
==========================================
- Coverage   93.47%   93.46%   -0.01%     
==========================================
  Files         110      110              
  Lines       38846    39119     +273     
==========================================
+ Hits        36310    36562     +252     
- Misses       2536     2557      +21     

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[mcollina]

Adding the benchmarks somewhere will be good.

Also, lint is failing.

@KhafraDev I'm generically ok with these improvements. However it's quite substantial in the code changes. Take a look.