Plenty of sites do use client-side instrumentation, and so they do indeed benchmark their site on your machine, and then make decisions based on the results of those benchmarks (likely aggregated with many other benchmarks from many other users). Yes, sometimes JavaScript is used poorly in ways that make sites bloated and slow, but plenty of sites use it properly in ways that only make their sites better (collecting the appropriate data to confirm that what they are doing is making their site better).