The “reveal” style keeps the menu in the background, uncovering it as the main page content slides off the screen.
Going mobile first, you start out designing big and tight. With desktop first, you will have to decide on what gets to stay and what has to go.
Plenty of frameworks support responsive design, but I've been looking for one with a distinctively mobile first point of view.
Varnish cache speeds up websites, but can cause issues for content viewed on mobile devices. Configure Varnish properly for mobile and desktop browsers.