Even on cached page views and repeat views, rendering takes a fair amount of time. @Peter suspects the parsing of data URIs to be a possible bottleneck. Especially on mobile.
This is non-trivial to undo since we rely on these to some extend to avoid flashes of unstyled content (e.g. hover states, dialogs etc.). Normally, when browsers load stylesheets, referenced images are not downloaded until after the selector applies (at which point it's kind of too late). We may be able to work around this by using preloading or prefetching, however.
Let's start by investigating a static copy of our page with and without data URI embedding enabled in a stylesheet and run it through WebPageTest and others to compare rendering.
"Extreme Web Performance for Mobile Devices", Maximiliano Firtman, at Velocity 2015 Amsterdam