Use Debug Mode (Ctrl-F4, Ctrl-F5) to see what's causing performance loss. You can see how much memory images are using, how many collision checks are made every tick etc. Profile tab shows which event groups are most CPU-intensive.
For example, on my laptop "Level Building" group was using 17% CPU. After I changed event 35 from running on every tick to On Start of Layout, it dropped to 0%!
Also, search this forum and tutorials for performance tips, there have been several posts with lots of useful info.