When working on performance-sensitive code (query observers, hydration, render-triggering state, hot utilities), ensure you only do the necessary work and no redundant allocations/wrappers, while also preventing leaks.

Apply these rules:

• Avoid unnecessary network/UI work: don’t enable fetch/retry paths when data is already synchronously available—add/keep tests that assert loading-state transitions.
• Make change detection “minimal but correct”: compare the specific fields that can affect rendering; missing metadata/derived fields can silently block required recomputation, while incorrect comparisons can trigger extra recomputations.
• Eliminate repeated allocations in loops/hot paths: precompute data structures used by callbacks (e.g., create a Set once, not per filter iteration).
• Avoid redundant expensive computations: don’t call deep-compare/structural-sharing helpers twice when you can short-circuit.
• Wrap expensive logic once, not per-page/per-iteration.
• Always clean up long-lived resources (subscriptions, BroadcastChannels, event listeners) to prevent gradual performance degradation.

Example (single Set allocation in a hot utility):

function difference<T>(array1: T[], array2: T[]): T[] {
  const set2 = new Set(array2)
  return array1.filter((x) => !set2.has(x))
}

Example (wrap fetch once when a persister exists):

if (context.options.persister) {
  context.fetchFn = () =>
    context.options.persister!(fetchFn as any, {
      queryKey: context.queryKey,
      meta: context.options.meta,
      signal: context.signal,
    })
}