See example: http://output.jsbin.com/jexixam/quiet
Initially, before the "Slow" font-face has been loaded or timed out, the sample text takes about 2.5-3x more space to layout (on iOS, it takes about 8-9 lines and once the font is loaded/timed out, it takes only 3 lines). In other words, the estimate is too rough.
It appears that Safari assumes mono-wide square characters, which is very uncommon. It might be a better option to use a fallback font to estimate character layout before the desired font is loaded (as in `font-family: Slow, Arial` where "Arial" is already available as the fallback). This would also reduce the work the browser has to do in case of timeout.
I think I fixed this in r216944. Can you see if the problem still reproduces on a nightly build?
Does appear to be fixed in nightly. Thanks for the fix!