The maybe you stop the clock when the user navigates away from the page. If it takes 30s to read the content without ad-loading and 40s to read it with ad-loading, then by subjective user time, the page is 10s slower. That may be because the ads take 10s to load, or because the distraction causes the user to read at 75% normal speed, but it's still a measurable result from a single variable change.