In some instances, showing nothing for a few seconds can be preferable. Some fonts are very different in size and that web font could flow a lot differently than what the fallback is. If you start reading and the font changes halfway through the sentence, it can be very jarring and slow you down as you try to find your place again.

A lot of the problem is poorly implemented web fonts that are not optimized, or web designers going crazy and including 6-8 different font/weight combinations. Having half a meg of web fonts download is no different than using way too many images that are not compressed enough.