Unless someone has either gone totally overboard on the number of fonts or set their site up very badly, this is largely a non-issue in most markets today.

The total weight of all web fonts on any site/app I've worked on recently was typically less than the largest single image on the home page, and loading those fonts from a well-configured CDN rarely took more than a few tens of milliseconds and happened in parallel with everything else.

Of course you could get a FOUT if you are using an unreliable Internet connection and it drops out at just the wrong time. But then, you could equally well be missing important images or seeing a layout reflow when a stylesheet finally loads in that environment as well.