I've extended the simple sideways scrolling project to see how difficult it would be to incorporate an enemy...
Can you let me know if this lags now on your PCs? Also, I needed to alter some of the scripts a little to make it work... I might back some of this out or change it around till I find the best way :)
*** Updates ***
* Added a moving platform sprite.
If you have any ideas, or want me to extend this project with more features then let me know.
What this project has: Horizontal smoothed out Parallax Scrolling, Simple player / level Collision Detection and fixing, Gravity, Smooth variable height Jump.