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design-editor-performance

What each screen costs on Design's canvas, the budgets that bound it, and how to prove a change keeps them. Use when changing culling, previews, live editors, zoom/pan/drag, DesignEditor hooks or caches, or when Design lags, flashes, or grows memory.

templates/design/.agents/skills/design-editor-performance/SKILL.md

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# Design Editor Performance

The general rules (closure chains, cache budgets, view swaps, measuring after
GC) are in the root `performance` skill, §11. This is what they mean on the
board, where one user session holds dozens of multi-megabyte screens.

## What a screen costs

| Shown as | Runs in | Costs | Bounded by |
| --- | --- | --- | --- |
| Live editor | same-origin iframe, main thread | ~20 MB and ~380 ms of main thread to boot | 32 in the pool, 4 booting at once, 3 kept warm through a zoom-out |
| Static preview | sandboxed `srcdoc` iframe, preview process | its ~2 MB `srcdoc` string in the main heap; the preview process renders one at a time | 64, nearest first, then screens already seen |
| Placeholder | nothing | nothing | everything else |