1. If your application is unused for a while it gets unloaded (from the server memory).
2. On the first hit it gets loaded and stays loaded until some time passes without anyone accessing it.

This is done to save server resources. If no one uses your app why keep resources busy and not let someone who really needs use them ? If your app has a lot of continous traffic it will never be unloaded.

stackoverflow link for more https://stackoverflow.com/questions/2606190/why-are-my-basic-heroku-apps-taking-two-seconds-to-load