I measure with the Profiler first, then stabilize references with useCallback and useMemo, wrap re-rendering components with React.memo, virtualize long lists, lazy-load images, code split routes, and debounce expensive inputs. I measure again to confirm each fix helps.

Use semantic HTML, give interactive elements accessible names, ensure keyboard accessibility, manage focus for modals and menus, ensure sufficient color contrast, use ARIA only when needed, and test with a screen reader.

Mobile-first with breakpoint prefixes or media queries, tested on real devices. Use flexible layouts with flexbox and grid, responsive images, adequate touch targets, and plan mobile navigation in the layout from the start.