Nobody denied that it is slower in theory, but I still claim that you won't be able to measure the difference on an average web page. So time is not an excuse for not implementing this (and this was the original question).
By “non-negligible” I meant something like 2–10 times slower (you’d have to test to be sure... I tried searching around but couldn’t find any direct speed comparisons). For typical pages it’s not going to matter, but if you have a page like the HTML5 spec, where it takes a half a second to resize a window now, it’s going to be noticeably slower using a more sophisticated paragraph composer.
Keep in mind, Safari doesn’t even do kerning, because they’re afraid it’d be too slow. (Also a ludicrous excuse for desktop hardware and typical web pages, IMO.)
And the Safari guys also use the same excuse (“it’s too slow”) for not doing real color management with CSS/HTML colors.
I would like to have two browsers of the same type, one with high(er) quality typography and one as it is now. And then compare the speed. That would be interesting.
I imagine it would have to make a fairly significant difference in rendering quality in order to make any speed losses worthwhile. Death by a thousand cuts and all that.