Search optimization page should be broken down by type of optimization

inspired by https://github.com/elastic/docs-content/pull/6932#discussion_r3422512719

language-specific optimizations are mixed in with hardware optimizations. let's reorg this page so we have the hardware optimizations, a clearly language-identified section, and links to other language-specific optimization sections like the ES|QL optimizations page. this will stop people who use other languages/endpoints from starting wild goose chases

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## Elastic Docs AI Scoping 🤖

Docs issue scope

### Summary
The issue asks for a structural reorganization of the "Tune for search speed" page (`deploy-manage/production-guidance/optimize-performance/search-speed.md`) to separate hardware/infrastructure tuning from language-specific (Query DSL) optimizations, and to add cross-links to language-specific performance guides. The linked PR #6932 adds a minimal cross-link to the upcoming ES|QL query performance guide (elasticsearch#150360, not yet merged) but doesn't reorganize the page; the issue was opened after the reviewer noted at line 370 that ES|QL (and other non-DSL) users have no signal for which sections apply to them.

### Request accuracy
Accurate — the current page mixes infrastructure-level tuning (filesystem cache, readahead, hardware, replicas) that applies to all query languages with Query DSL-specific techniques (document modeling, `index_phrases`, `index_prefixes`, `constant_keyword`, `avoid scripts`, `search rounded dates`, `pre-index data`) that do not apply to ES|QL or other query interfaces.

### Next action for author
Restructure `search-speed.md` under clearly labeled H2 groups (e.g., "Infrastructure and hardware" and "Query DSL optimizations"), and add a language-specific callout or section near the top that links to ES|QL and other language-specific performance pages.

### Impact: Medium

### Scope boundary
The ES|QL performance content itself is out of scope here — that lives in `elasticsearch#150360`. This issue is only about reorganizing the existing `search-speed.md` page structure and adding cross-links.

### Recommended documentation targets

| Page | URL | Action | Impact | Confidence | Why this page? |
|------|-----|--------|--------|------------|----------------|
| Tune for search speed | https://www.elastic.co/docs/deploy-manage/production-guidance/optimize-performance/search-speed | Update existing page | Medium | High | This is the page with the structural problem — needs H2 grouping separating infrastructure from Query DSL-specific sections, plus a language-specific callout or intro blurb |
| ES performance optimizations (parent) | https://www.elastic.co/docs/deploy-manage/production-guidance/optimize-performance | Review only | Low | Medium | The parent page lists `search-speed.md` alongside `indexing-speed.md`; a short annotation noting that `search-speed.md` covers infrastructure tuning and Query DSL may improve navigation |

### Recommendations

1. **Update `search-speed.md`**: Add H2 grouping headers to separate the ~10 infrastructure/hardware sections from the ~10 Query DSL-specific sections. Suggested groups:
- "Infrastructure and hardware tuning" (filesystem cache, readahead, faster hardware, force-merge, warm-up, index sorting, replicas, preference, timeout, open search contexts)
- "Query DSL optimizations" (document modeling, search-as-few-fields, pre-index data, keyword mapping, avoid scripts, search rounded dates, Search Profiler, index_phrases, index_prefixes, constant_keyword)
2. **Add a language-specific callout or intro blurb** near the top of `search-speed.md` explaining that the Query DSL-specific section applies to Query DSL and is not relevant to ES|QL, and linking to the ES|QL performance guide (`(reference/redacted) once elasticsearch#150360 is merged.
3. **Coordinate with PR #6932**: The ES|QL cross-link section already being added at line 370 by PR #6932 may be superseded by or incorporated into this structural reorganization; the two changes should be reconciled.

### Notes

- The infrastructure sections already carry `{applies_to}` YAML blocks for self-managed/ECK — those should be preserved during the restructuring.
- The "Query DSL optimizations" grouping sets a pattern for future language-specific sections (e.g., if a Painless or EQL perf guide is added later).
- The Search Profiler section works for Query DSL queries; ES|QL has its own profiling approach in the ES|QL guide — this is worth noting in a callout.
- Dependency: the ES|QL cross-link in recommendation 2 is blocked until elasticsearch#150360 merges, but the structural reorganization (recommendation 1) is independent and can proceed now.

> Generated by [Issue Scope Analyzer](https://github.com/elastic/docs-content/actions/runs/27636548024) for issue #6943 · 184.9 AIC · ⌖ 18.2 AIC · ⊞ 29.3K · [◷](https://github.com/search?q=repo%3Aelastic%2Fdocs-content+is%3Aissue+%22gh-aw-workflow-call-id%3A+elastic%2Fdocs-content%2Fgh-aw-docs-issue-scope%22&type=issues)

Docs RICE prioritization

| Dimension | Score | Rationale |
|-----------|-------|-----------|
| R · Reach | 3 | 2,154 sessions/90d on search-speed — solid traffic; real audience broader than pageviews since ES\|QL users land here and get misdirected by Query DSL sections |
| I · Impact | 2 | Marginal — no active harm, users are confused and waste time chasing inapplicable advice; fix is orientation/navigation improvement |
| C · Confidence | 1 | Scope and value fully clear; PR already implemented and validated |
| E · Effort | 0.25 | 2–3 days (restructure, style fixes, correctness audit, SEO validation, applies_to validation) |

**RICE = (3 × 2 × 1) / 0.5 = 12 → P1: High**
Scored: 2026-09-11

See the [prioritization rubric](https://codex.elastic.dev/r/docs-content-internal/processes/cross-doc-prioritization-rubric) for scoring details.