Waterfall diagram showing impact on latency and resource utilization
The impact on page performance is severe. It is clearly noticeable even on a high-bandwidth, low-latency connection. The attached image shows the font requests saturating bandwidth, then CPU as they are retrieved and rendered.
More details here: http://www.webpagetest.org/result/140111_D1_TEV/1/details/
WebKit browsers hide text until the font is available. This causes the interlanguage links for which ULS is overriding the system default font selection to appear in one font, vanish for several seconds, and then re-appear in a different font. I recorded this on my laptop: http://www.youtube.com/watch?v=J3Wd7d8Y7wE. Note that I am using a bleeding-edge browser version and connecting to the internet via a broadband connection.
In my assessment, the frequency and regularity with which this issue has recurred makes it clear that ULS's strategy for loading web fonts is fundamentally misguided. I do not consider a selector exemption to be an adequate solution to this bug.