The mobile team has been working on some significant performance improvements. One of the trickier yet hopefully most rewarding improvement is to remove the need to vary the mobile varnish cache by X-Device header for regular page HTML. Updated code is already live in production[1], however until we set $wgMFVaryResources = true, pages in the mobile varnish cache will still be varied by X-Device header as they have been all along.

We've enabled $wgMFVaryResources on testwiki[2] and have been doing some sanity checking to ensure major devices are still working correctly. During our regular weekly MobileFrontend deployment on 16 April, we plan to enable this on mediawiki.org for further, crowd-sourced testing. More details to follow. Assuming things go well, we will go ahead and make this change live everywhere, hopefully during our deployment on 23 April.
Arthur Richards