In a comment on my post about optimizeLegibility, Marco Arment points out that there are significant, and even fatal, performance problems with this text-rendering mode on mobile devices. Marco Arment writes:
I did some testing with Instapaper to determine approximate optimizeLegibility performance limits. A 5,000-word article in Instapaper for iOS, for instance, will only use optimizeLegibility on devices with an A5-class or greater CPU. To avoid problems on older iOS devices, I wouldn’t recommend using optimizeLegibility blindly and unconditionally on any pages longer than about 1,000 words. And I wouldn’t recommend enabling it on Android at all.
So for now it seems like it’s a good idea to be conservative with the use of this property, using it sparingly and avoiding it altogether for older mobile devices. If in doubt, use it for headings only.