A simple (to install) solution for this is Google PageSpeed Module - https://developers.google.com/speed/pagespeed/module/. It's like a proxy in front of your website that will serve optimized / minified / concatenated versions of the theme and such. It inlines small stuff, resizes images where needed, all that good stuff.

I don't have it on my current webserver though, I'm not comfortable manually having to compile nginx, it seems like there's a lot that can go wrong there.