Measure Reliably

Extensions run content scripts on your page, on your main thread, inside your measurements. Ad blockers rewrite the DOM, password managers scan every form, React DevTools instruments each render — all of it lands in the same numbers you are about to draw a conclusion from. The damage is not that the timings are slower but that they are noisy: when run-to-run spread is larger than the regression you are hunting, the profile stops being evidence. Measure in a clean profile or incognito with extensions off, take several runs, and compare spreads rather than a single number.