For ten people, shaving 0.3 seconds per page load would save 3 seconds of their total shared lives from sitting and watching a page load. For a million^ people who load web pages at least once, you technically have created an opportunity to save 0.3 * 1_000_000 = 300_000 seconds of cumulative lifetimes across them, which is 3-4 days of reality.

Practically, I'm not sure how I feel about this metric on the 0.3 second level, but I sure do understand it at the 5 second level. I've stared at useless pages that are stuck on some tracking JS so many times I can't count, so if those stop making me stuck, that's invaluable once and multiply that win by a million.

But that's just time. If you also consider the resources that are needed to execute the page JavaScript, we're probably saving megawatts of power per interval (totaled across all Firefox users) with each small efficiency gain. The changes that are "parallelize all the work" may cost more, less, or same amount of power; the ones that are "use fewer CPU cycles" will definitely cost less power. And so there's a very real theory that we could probably measure how many gigawatt^-hours Quantum has saved the world to date, which appeals to me greatly :)

^ I arbitrarily picked "a million" and "gigawatts". This does not offer any guidance whatsoever on specific or approximate user counts.

EDIT: My UPS says that my entire desk has a load of ~75 watts @ 120V, times a million^ users = 75 megawatts. I haven't the faintest idea how much power it takes to evaluate Javascript inefficiently, but saving 0.3 seconds of any amount of power times a million^ users per page view quickly balloons into reality-impacting figures.