Computer Loads Games Very Slowly

Whenever I load into a game of league on my desktop mac, it takes between 5 and 10 minutes. Closing applications in the background helps, but even with nothing else open it generally takes at least 5 minutes to load. I play on low graphics with shadows off, and I usually get over 200 fps when in game. I downloaded League of Legends onto my laptop, also mac, and even though it got 40ish fps, and had default graphics, it loaded into a custom game in about 2 minutes. Any ideas on why this happens?

Specifics about my computer: iMac, 21.5 inch, Mid 2011 Processor: 2.7 GHz Intel Core i5 RAM: 4GB 1333 MHz DDR3 Graphics Card: AMD Radeon HD 6770M 512 MB