Improve custom-host HTML minification performance while preserving output semantics

[ryoppippi]

Measured optimisation opportunity

After removing embed rendering from client island discovery (see #1370), the custom-host closeBundle phase remains around 3 seconds on ryoppippi.com with @ox-content/vite-plugin 3.1.1. Temporary timing probes identify HTML minification as a measurable part of this cost; this is an optimisation request, not evidence of a particular regression.

Warm local build, 402 client modules and 428 custom-host outputs:

Phase	Observed duration
Host loader server	25–41 ms
Load host module	141–168 ms
Load/prepare routes	1.42–1.56 s
Render routes	169–253 ms
Output planning	157 ms
Self-hosted assets	119 ms
HTML minify + writes	573 ms
Loader shutdown	251–371 ms

An A/B run changing only custom-host build.minifyHtml:

• Enabled: coordinated output writer 974 ms; complete pnpm exec vp build subprocess wall time 4902 ms.
• Disabled: coordinated output writer 389 ms; subprocess wall time 4123 ms.
• Vite client build remained about 0.75–0.77 s.

These are single-machine diagnostic measurements, not a controlled multi-run benchmark. The writer difference is about 585 ms; total process time also varies in other stages. Production minification remains enabled; disabling it is only an experimental control.

Current implementation

Verified current source: https://github.com/ubugeeei-prod/ox-content/blob/main/npm/vite-plugin-ox-content/src/html-minify.ts

minifyHtmlOutput uses html-minifier-terser with Terser and clean-css callbacks. The custom-host output writer invokes it for every generated HTML route. Promise.all over route writes does not itself move JavaScript CPU work off the main thread.

Request

Please profile/optimise this production path while preserving the existing semantic guarantees, including hydration comments, whitespace, embedded JS/CSS and error handling. Potential approaches to evaluate include reuse of identical inline-script/style minification results within a build, or a faster compatible backend. These are investigation suggestions, not verified fixes.

A public supported fast path would let downstream custom hosts retain production output quality while reducing total SSG build time. Reproduce using a custom-host site with dozens of HTML pages, transformHtml: false, and minifyHtml: true; compare writer/total timings with minification disabled.