Hi-ReS-Stats (formelly Mrdoob stats) provides a simple info box that will help you monitor your code performance. It can tell you the following:

• FPS Frames per second, how many frames were rendered in 1 second. The higher the number, the better.
• MS Milliseconds needed to render a frame. The lower number, the better.
• MEM Memory your code is using, if it increases per frame is VERY wrong.
• MAX Maximum memory the application reached.