Love ur Desire and Prevent Lags
Lag is typically a result of Android working really hard to do all the things you want at the same time. You may be right that an application may be hogging the CPU or even the RAM. Try to keep the number of applications/processes you do to a minimum (e.g load a web page, while sending an email, while opening contacts, etc.).
If you want to attempt to resolve the problem let's first assume that it's an application that's causing the problem.
Go to Settings (from Home Screen) --> about phone --> and check to see if any applications are on the list. If the application has a decent amount of battery usage from it (>5%) you may want to investigate the app further.
Then go to Applications (from settings) --> running services. This will let you know what apps have been stored in memory (RAM) and how much space they're taking up. When an application takes up more RAM then it should, it can cause our phone to work harder to get rid of apps not in use (this is how Android multi-tasks).
Ok let's assume this isn't an apps fault it may be time to clean your system cache. Have you ever noticed when you get a new computer, it's super fast, but as the years go on, your PC gets slower and slower. Sometimes things get stored in a cache which can slow down the system. To clear the cache on your android all you have to do is reset your phone to factory defaults. WARNING THIS WILL DELETE ALL OF YOUR APPLICATIONS you can of course re-download them, however it's not guaranteed that any of the information you have saved in the apps will persist. Anything saved on your SD card will remain unaffected.
Once you get everything downloaded and back to normal, you should notice your phone is pretty zippy again. However reseting your phone can be a pain and should only be done if you really can't pinpoint an application(s) that is responsible.