As I could not work on my sidescroller on vacation, I've worked on two other things: 1) Screenspace reflections, that means no additional rendering (except a depth view) is involved. They can be attached to arbitrary surfaces but have many limitations (only what is on screen can be reflected and so on) and, as a work in progress, there are still many bugs.
(a screenshot from my laptop, runs at 1024fps on my desktop)
2) A pretty fast 2D collision system that supports boxes, spheres and convex polygons with GLIDE support for my future sidescrollers:
