The Doherty threshold (~400 ms) describes how sub-second response preserves attention and perceived fluency. Beyond it, workflows feel sluggish even if “acceptable”.
Perceived performance
Skeletons, optimistic UI, prefetch, and streaming content beat raw spinners — as long as truth catches up quickly.
Backend and batching
Aggregate API calls, cache hot reads, paginate thoughtfully, and move heavy work async with clear job status.
Interaction budget
Animation and motion should stay under interaction latency goals. Long easing curves on frequent actions add fatigue.
Practices
- Lighthouse and RUM budgets in CI.
- Inline critical CSS; lazy-load below fold.
- Debounce search; cancel stale requests.
Common pitfalls
- Blank screens while JS downloads.
- Giant client bundles for static content.
- Ignoring mobile CPU throttling.