If you are new to AE then you just have to get used to working with slow previews and learn how to get things done efficiently. On the same machine I can usually work at least twice as fast as most AE artists with just a few years experience because I never try and do full rez full frame Ram Previews of completed composites. Let's define a few terms here. Blocking is where you arrange the elements of your composition on the stage (comp window) and move them around. This could be text, graphics, or other things that you are going to animate. Effects are things you do to a layer that change the appearance of that layer's color. Distortions are things you do to a layer to change the shape of a layer. This includes the obvious things like Mesh Warp and Liquify, but also includes things like displacement maps. Particle effects are effects where you generate things like smoke, dust, or puzzle pieces. The list goes on and on.
Here's how I work on most of my projects. First I add my elements to the stage. This includes things like graphics, text, and footage. Second I do the blocking and animate the movement of my elements. Most of the time this is done at 25% or 50% zoom factor in the comp window and the resolution of the comp window is set to Auto. I preview each move of each element and when the move is what I want I am done with the blocking. Ram Previews are very fast in this phase. If I have to I even use Draft 3D for this phase of the animation.
Now it's time to add effects. I do this a layer at a time and I check just a few frames at 50% to 100% zoom factor. I usually do not do any Ram Previews at this point. There is no need because I already know how the motion looks. Then I add any distortions, particle effects or Lights to a 3D scene. Here again, except for particle effects, I don't do any ram previews.
The final step is to turn on motion blur and check a few critical moves at full rez and 100% Zoom factor. Occasionally I will pick a 4 or 5 second section of the project and do a 50% Zoom Factor RAM preview and on very rare occasions I'll preview one or two critical sections of the animation at 100%. I NEVER ever try and do a ram preview of a Full Rez comp at 100% Zoom Factor that is more than 5 or 10 seconds because I already know how the blocking works and I know how the effects look and I know how the distortions work. I can't afford the time to sit around and wait. I send the Project to the Adobe Media Encoder to have it crank out the first test render and I move on to the next shot in AE. IMHO, working any other way is a waste of time and it also hurts your creativity. How does it hurt your creativity? I've been making movies for 40 years and I've never met anyone that didn't get a better idea of how their project is working after stepping away from the details for at least 10 or 15 minutes. Move on, focus on something else, then go back and look at what you've done and tweak it then if you need to.
Even if AE would do full composite Ram Previews in real time I probably wouldn't spend very much time looking at the whole project with everything done. I'd just throw the project at the encoder as soon as I thought I had something that worked, move on, then look back later to see if anything needed fixing. That's the way I work in PPro even when I'm editing. I concentrate on the performance and the cuts, then move on and go back to the scene later to see if it can be improved.
If you're new to AE, there are likely a lot of things you're doing that aren't the best way to work. Rick has mentioned some good workflows, I will suggest that you start here: Getting started with After Effects Getting a solid foundation in how to work with AE will help prevent a lot of frustration.