[Dom Panico]

I just got after effects and my new macbook pro with an i7 processor and 4 gb of memory. whenever i open after effects and import any clip of footage i have from my 1080p camera all HD, i press space bar to play it. And it goes very slow. I assume this is because it is rendering, but i mean it goes REALLY slow. more than 30 seconds and it slows down my entire computer in general. Once i close the program my entire computer studders because it is having a hard time to catch up from the Really long rendering job from just importing and testing the footage. Why is that?