Well, I've done a bit of experimenting with different cameras and different settings - the point at the time was to figure out the best method/settings for me to use. My cameras shoot both in 1080p and 720p and I've started to use the 720 mostly because of the file size (even with 720, I can easily pile up 100 - 150 GB of various project files for an hour project) and I also noticed that I simply could not tell the difference on my TV...
Video rendering is the single most intense workflow for the processor - you need the best/fastest processor and as much RAM as your machine can handle. Even then, it is still slow.