Technically, this is postponed on #1757550: [meta] Convert core theme functions and templates to Twig, but hopefully we could get a good start well before then.
But we need to understand the performance implications of Twig, and hopefully mitigate some of that prior to merging it in.
I'm currently aware of the following performance-related Twig issues, but there may be more:
- #1922304: Remove TwigReference objects in favor of a high speed implementation by using NodeVisitors more cleverly
- #1815250: Declare render variables in PHP and remove superfluous twig_render() calls
- #1938430: Don't add a theme hook class in template_preprocess()
- #1979290: Add static caching to Twig_Environment::getTemplateClass()
There's also some really nice profiling data at #1818266: [meta] A secure theme system (with twig), but that deals with the auto-escape system and this issue's about more "raw" data just in template/theme function conversions before we start adding in more doo-dads.