In my experience, you can see noticeable performance losses pretty quickly on mobile devices in as few as 9 on-screen icons, especially in cases of animated elements.

edit: caveat for animated elements.

That fits with what the Sencha team reported for iOS 7 - big wins for static SVG but bigger losses for animation:

http://www.sencha.com/blog/the-html5-scorecard-the-good-the-...

Look at it this way, everywhere on your screen you see repetition is an opportunity for a new, simple symbol that can be represented with fundamental glyphs.