Um, I think you're assuming that AE ought to work like a video editing application -- which it doesn't -- which then leaves me to think that you ought to get a good look at this:
> Even when it loads, everytime I make a small change, the green disappears and has to load again.
That is normal. Each time that you make a change, the images need to be re-rendered.
Follow the link that Dave gave you, as well as this one for resources about making After Effects work faster: http://adobe.ly/eV2zE7