Computer resources are the gasoline for running applications and, therefore, they should be one of the main worries of a game programmer. Most software come with a system requirements list that draws the line between a computer that runs it properly and another one that doesn't. Testing your application on a wide range of target systems is a good practice but it is not the only one.
VisualVM is a free tool to view real-time detailed information about Java programs through an intuitive interface. It brings with it monitoring charts, threads breakdown, and some other utilities to profile your application.
It can help you to optimize slow games, improving the game experience, or even widen your target audience.