Firstly, linux is rarely using as much RAM as it appears to be. When something is loaded in RAM, after you exit the app the kernel won't release the memory assigned to it so then later if you want to use that again everything is already in memory waiting to go. It's actually very smart, if something else asks to use that memory then the kernel will free it so other apps won't miss out. You can use 'free' to see exactly how much RAM is being used and how much is taken up in caches etc.
CPU usage, not sure but open a terminal and run 'top'. It'll tell you what processes are using the most CPU time and you can do more hunting from there once you know what the culprit is