> Stop the clock when, in your opinion, you think the page is ready to read.

Even that is not enough. A number of sites (especially ad-heavy sites on mobile devices) look ready to read, but as soon as you try to scroll, are jerky and semi-unresponsive. It's particularly a problem on ad-laden sites of small news publications. Blocking ads sometimes mitigates this, though not always, as some sites insist on using Javascript to implement "fancy" scrolling which inevitably breaks.

(Semi-related sidenote: please, for the love of God, don't use Javascript to hijack browser scrolling. It's almost certain you will not get it perfectly right and some users will suffer. It's almost never worth it. This is the #1 reason I end up having to switch to Reader Mode in Firefox).

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.

You're going to get a whole lot of noise in your signal from readers who get sick of waiting for/looking at the ads and leave early.

> Stop the clock when, in your opinion, you think the page is ready to read.

And then the page completely re-flows because some big advertising asset popped into existence further up that you weren't expecting.

I'd still question the usefulness of this rather arbitrarily number.

The only reason I can think of where I'd be interested in how quickly people can navigate a site without having loaded all the content yet, is when optimizing a site for people with really slow/bad internet connections. But that seems like a whole other topic that has barely any influence on the goal of the comparison.