How does the Performance panel in DevTools help diagnose a janky UI?

Recording a performance profile captures a timeline of scripting, rendering, painting, and layout work, letting you spot long-running JavaScript tasks, layout thrashing, or excessive re-renders that block the main thread and cause dropped frames.

// Programmatically mark points to correlate with the profile:
performance.mark('start-render');
renderList(items);
performance.mark('end-render');
performance.measure('render', 'start-render', 'end-render');