It is very frustrating when loading up a website and seeing a mostly blank page because the fonts are still downloading. This is especially frustrating on a slow internet connection or on mobile.

I'm sure that more people will discover and adopt best practices to improve performance and overall user experience over time, but right now I'm noticing this slowdown very frequently. - see http://www.igvita.com/2014/01/31/optimizing-web-font-rendering-performance/ for some notes on best practices

To me, speed is still the most important feature of a website, and it's shocking how bad this experience often becomes when web fonts are added to the equation.

This is one of the biggest problems I have with the current web because of how frequently I'm noticing this.