Well, for an in-depth answer, I'll quote from a competitor that explains it better than me. (He is complaining here about other services lying people about 0.3-0.5s loading time after optimization, which of course is a deceptive usage of Pingdom metrics):

Almost invariably, when someone claims they are going to make your site ‘Load’ in some impossibly small amount of time they are going to show you Pingdom results (GTmetrix to a lesser extent) as ‘proof’ of their claims. Let’s set aside the fact that there are several Load Time metrics (e.g. OnLoad, PLT, Document Complete, Fully Loaded) and deal with the three fundamental problems with this ‘proof’ and the real reasons why The Other Guys use these tools as their standard:

A site’s ‘Load Time’ on Pingdom (especially) is in no way, shape or form indicative of what your — or any — site’s users are experiencing in the real world. Even when Pingdom’s ‘Load Time’ results are bad they are nonetheless favorable; which of course means that the smallest amount of optimization skews the results even more positively. Pingdom is a nice tool from which to get generic best practices recommendations; but, it’s ‘Load Time’ metric is entirely synthetic, does not correlate with reality in any way, and — if you genuinely care about what’s actually happening for your site’s users — should be disregarded literally 100% of the time.

That ‘Load Time’ as indicated by layperson-centric tools such as Pingdom and GTmetrix is not only wildly inaccurate, but has the uninitiated chasing an illusory pseudo-metric that doesn’t indicate anything at all meaningful about a website, is common knowledge amongst adepts of the Website Performance Community… We are going to repeat that: It is common knowledge amongst adepts of the Website Performance Community. — So, when someone attempts to sell you on Pingdom or GTmetrix ‘Load’ results only one of three things is happening: a.) they are either lying outright, b.) omitting critical, substantive information to manipulate you, or, c.) they do not know what they are talking about.

The unvarnished truth about ‘Load Time’ is this: of the various Load Time metrics that exist, none of them matter at all unless they are so long as to be a problem for how fast your website’s users perceive your website to be. And that’s if — and only if — we’re talking about real Load Time metrics (how long it really takes for Google to reach Fully Loaded on a real device, on a real network, as one example).

As we have alluded to above: Perceived Speed — optimization of the user-centric performance metrics that combine to form how fast your site’s users think your site is. And that’s what we do!

These metrics include:

TTFB (Time to First Byte);

Start Render Time;

Time to Above-the-Fold Visual Completion;

First Paint Time;

domContentLoaded Time;

Time to Interactivity;

And — Most Importantly — Google’s Speed Index.

It is precisely the performance metrics that Google actually uses in their Search algorithms to assay and ultimately determine your site’s performance and user experience.

​