Performance Optimization and Profiling

Browser devtools include a "Performance" tab that records exactly where time is spent while a page runs — script execution, layout, painting — instead of guessing.

Common, high-impact fixes once a bottleneck is found: avoid causing a "layout thrash" (repeatedly reading and writing to the DOM in a loop), debounce expensive handlers (like a resize or scroll listener), and lazy-load anything not needed immediately.

Optimize what the profiler actually shows is slow — don't guess.