Performance — Keep Loads Fast

Rule

Treat every list, every read, and every page load as a latency budget. Two things dominate it: how much data crosses the wire, and how many round-trips and table scans it takes. On a hosted/serverless SQL backend each query is a network round-trip, and an unindexed filter scans the whole — often shared and growing — table. So default to projected columns, indexed hot-path queries, and parallel/batched fetches. These rules are provider-agnostic: they hold on SQLite, Postgres, or any managed SQL backend.

This skill is about the data and load path. See the storing-data skill for the schema and migration mechanics it references, and the real-time-sync skill for how updates already reach the UI without polling.

1. Project columns — never SELECT * on a list

A list/index query should select only the columns the list actually renders.

• Never return heavy columns in a list: large JSON/text blobs such as document bodies, rendered HTML, config/layout/spec/data/tracks, tool results, or base64 attachments. Pulling them for every row is the single most common cause of a slow list.

• Heavy/full columns belong on the single-item GET/detail path only.

• Need a preview from a big column? Select a truncated substring at the DB, not the whole column — and it stays portable:

  // Drizzle — project, and truncate the heavy column for the preview
  const rows = await db
    .select({
      id: docs.id,
      title: docs.title,
      updatedAt: docs.updatedAt,
      // substr/length work on both SQLite and Postgres
      preview: sql<string>`substr(${docs.content}, 1, 400)`,
    })
    .from(docs)
    .where(accessFilter(docs, docShares))
    .orderBy(desc(docs.updatedAt));

• After narrowing the projection, update the row mapper and its return type so a dropped column is provably unused on the list path. If the list genuinely renders a heavy column (a thumbnail, an inline preview the UI shows), keep it — don't break behavior to chase a payload win.

2. Index the hot paths

Indexes are added through the versioned migration array in server/plugins/db.ts as CREATE INDEX IF NOT EXISTS … — not through a schema-level index() helper (the framework applies indexes via migrations; see the storing-data skill). Add an index for any column a hot query filters or sorts on. The recurring ones:

• Ownable tables → (owner_email, org_id, <the list's ORDER BY column>). Access scoping filters by owner/org and lists sort by updated_at/created_at.
• Shares tables ({resource}_shares) → (resource_id, principal_type, principal_id). Access checks run correlated EXISTS subqueries against these on every list.
• Child / foreign-key columns used to load children (e.g. responses.form_id, comments.parent_id, an events log's *_id) → index the FK, plus its sort column when the children are ordered. An unindexed FK means a full scan of the child table on every parent open. A foreign-key reference does not create an index automatically — add it explicitly.
• Status-filtered lists → match the real WHERE, e.g. (owner_email, status) or (status, <sort>).

Keep index DDL dialect-agnostic and idempotent:

CREATE INDEX IF NOT EXISTS forms_owner_org_updated_idx ON forms (owner_email, org_id, updated_at)

No DESC, no partial WHERE, no provider-specific syntax — it then runs on SQLite and Postgres alike, is safe to re-run, and applies on next startup. Indexes mostly bite as data grows and on unbounded child tables (a seq-scan of 10 rows is instant; of a shared, ever-growing log it is not), so index the growing tables first.

3. Don't fan out queries — batch and parallelize

• No N+1. Never loop issuing one query per item. Load children for many parents in one inArray(child.parentId, ids) query, then group in memory.
• Count in SQL (count()), never "select all rows then .length".
• Parallelize independent queries with Promise.all rather than sequential awaits — each await is another round-trip.
• Prefer one composed endpoint over several dependent calls.

For provider wrappers, inspect the upstream API before building a list-then- enrich flow. Prefer the richest endpoint that can apply the real filters and return the needed associations or participants in one paginated operation. Cursor pagination is already serial; adding a serial detail/enrichment request to every page doubles its critical path. Exhaustive records belong in corpus recipes or data programs with explicit coverage, not one agent tool call per page or item.

First-class provider actions should represent one stable conceptual operation. Keep arbitrary endpoint, filter, and pagination access in the provider API substrate; do not turn a convenience action into a capability ceiling or duplicate the provider transport, auth, quota, and cache implementation.

4. Avoid client-side waterfalls

• Don't gate query B on query A's result unless B truly needs it. Fire independent useActionQuery / useQuery hooks in parallel; never make the loading skeleton wait on a serial chain.
• Load the visible page from one read where possible, and lazy-load secondary / below-the-fold data after first paint.

5. Poll cheaply; compute once

• Updates already reach the UI through the real-time-sync skill (useDbSync / SSE). Don't add an aggressive refetchInterval that re-runs a heavy list/read every couple of seconds. If you must poll, use a wide interval and a cheap endpoint.
• Never do expensive per-request work on a read that runs on every load/poll: re-rendering HTML/markdown, pretty-printing, re-parsing / migrating / normalizing / sanitizing stored JSON. Do that work at write time (store the result) or compute it lazily only for the caller that needs it. Reads on the hot path must be cheap.
• Data the UI doesn't display (export formats, alternate renderings) belongs in a separate on-demand action, not baked into the hot read.

6. SSR shell caching — load-bearing, do not undo

Every SSR HTML page and React Router .data response is one impersonal, public shell, hard-cached at the CDN and served identically to every visitor — logged in or not. This is the single biggest lever on first-response latency: one shared cache entry serves the whole site instead of a per-user render on every request. Adding private, no-store, Vary: Cookie, a session read, or an auth branch to the SSR path defeats the cache for every visitor, not just one.

If you're debugging a slow first response, check whether something re-personalized the shell before concluding the render itself is slow — the fix is client-side data loading after the shell paints, never per-user SSR. See the authentication skill for the full model and guard:ssr-cache-shell plus ssr-handler.spec.ts (packages/core/src/server/ssr-handler.ts) for the enforced contract.

Never route mutation-fresh reads through SSR loader data. Data that changes when a user acts belongs in an action, read from the client with useActionQuery / useActionMutation and kept live by useDbSync() polling — that path never touches the SSR shell cache. A useRevalidator() after a mutation re-fetches .data with a plain GET and can legitimately be served the cached copy. SSR loaders render the public shell; the client resolves anything that must be fresh.

The one supported knob is the deployment-wide AGENT_NATIVE_SSR_CACHE env var: unset/on keeps the default, off sends no-store, and a duration such as 30s / 5m shortens freshness. It is for deployments whose host does not purge its CDN on deploy, or whose loaders genuinely serve mutable public data. It changes cache duration only — cookies are still stripped before render, so turning it off does not make SSR personalized. There is deliberately no per-route or per-request override; that is how one visitor's payload lands in another visitor's shared CDN entry.

7. Big payloads and long lists

• Paginate or window unbounded lists (messages, responses, events, activity). Don't load the entire history on open; load a recent window and fetch older on demand.
• Don't store unbounded blobs inline in a row that a list/load pulls. Reference large content separately so opening the parent stays cheap.
• Never inline binary payloads in columns a list, poll, or view-screen summary reads. Images, PDFs, audio/video, archives, screenshots, and base64 attachments belong in file/blob storage; SQL rows should hold URLs, asset ids, storage keys, or opaque blob refs.
• Virtualize very long rendered lists on the client so off-screen rows aren't parsed/rendered every update.

Checklist — run before shipping a list/read or a new table

• List selects only displayed columns; heavy blobs excluded or substr-truncated.
• Every hot-path WHERE / ORDER BY column is indexed (owner/org/sort, shares resource_id, child FKs, status filters) via a db.ts migration.
• No N+1; independent queries parallelized; counts via SQL count().
• Client fires independent queries in parallel, not a waterfall.
• No heavy recompute on every read; no aggressive polling of heavy endpoints.
• Unbounded lists are paginated/windowed; large blobs aren't inlined on the hot path.
• SSR HTML/.data path stays session-blind and cacheable — no private, no-store, Vary: Cookie, or auth branch added to it.
• Mutation-fresh reads go through actions + useActionQuery, not SSR loader data.