BitCoins dynamically adjusts the computation needed in order to claim coins. It will just make the computation harder. Of course, you would get more than the other users with less CPU power.
Perhaps my PHP-design skills are outdated, but is it normal to have a 1500 line index.php containing a mixture of configuation code, PHP functions and HTML?
Also, don't you use any template system?
... I cloned the github project, only to find myself confused :)