mp4's are notoriously computation-heavy because they're so heavily compressed. That explains the processing part.
RAM Previews allow AE to use as much memory as is available to it to play back in real time. That explains the memory part.
Thank you for the quick response. Ok, but what does it store in those 4gb ? And also , which video format do you recommend for working with in AE? Because I record my videos in .MTS format , and these seem to be struggling as well.
EDIT : Just tried , .MTS videos use less RAM (in ram preview), and they play back faster.
There's a whole industry in recommending techniques for optimizing After Effects performance. I transcode all video to a common format before post production.