Well, here's something you need to know from Day 1:
After Effects is not an editing program.
It was never designed to be one, it does far different things than one, it behaves differently than one, and you use it differently than one.
You need to learn the basics about AE, or you'll experience nothing but frustration:
The good news: people who know their way around editing software quickly pick up on the differences, so it's not as daunting a task as you'd think to start from the beginning. But it is necessary.
In every compositing app I have ever used if you make changes you need to re-render. Some of the really expensive ones render in near real time, but that is the nature of compositing. After Effects is NOT an Editing App, never was, doesn't want to be.
You need to learn how to work with the program. I never do full rez Ram previews when I'm working on animations because animation is just motion and timing. Even 1/8 rez will tell you how the motion is working. When I'm grading color or carefully checking motion I do run the comp at 100% zoom factor and check a frame or two at a time. I still don't do ram previews. When there is a critical bit of business that I need to look at I may, on occasion, render that small bit so I can take a look at it. As soon as I have completed a comp I same it, load it into the Adobe Media Encoder to render a proof or a DI and move on the the next shot in the list because AE if for making shots not editing movies. When I have my shots edited I bring them into PPro or FCP and edit the movie. That's how you get things done. That kind of workflow has been around for more than 100 years. Pick the shots you want to do special effects, do your best to create those effects, send them to the lab (render) and when you get the answer print back, cut it into your movie.
The same workflow idea also holds true for movies. Edit you scenes together one at a time and not necessarily in order, then assemble your scenes to tell your story, then watch the movie to see how it works and if something doesn't then reorder the scenes or if the scene doesn't work then recut the scene. Starting at frame 1 and moving to the end in a NLE on a project that is more than a couple of scenes is actually a very inefficient workflow. Nobody that I know that has been in the business for more than a couple of years and is very good at their job works that way. It's just too cumbersome.
Every pixel-based compositor must re-render when changes are made (Nuke, Flame, Shake, etc.) This is not an issue unique to After Effects.
If you want to edit, use a video editing program like Premiere. If you want to do compositing or motion graphics, learn to work with After Effects.
Start here: Getting started with After Effects