perf: Reuse Nth index cache

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (3a95317) 85.02% compared to head (71ac2d9) 84.89%.

Files	Patch %	Lines
css-inline/src/html/element.rs	50.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #267 +/- ## ========================================== - Coverage 85.02% 84.89% -0.14% ========================================== Files 18 18 Lines 1670 1675 +5 ========================================== + Hits 1420 1422 +2 - Misses 250 253 +3 ```

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[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #267 will degrade performances by 50.46%

_{Comparing dd/reuse-cache (71ac2d9) with master (3a95317)}

Summary

❌ 2 regressions ✅ 3 untouched benchmarks

:warning: Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	master	dd/reuse-cache	Change
❌	big_email_2	3.5 ms	5 ms	-29.41%
❌	big_page	11.7 s	23.7 s	-50.46%