fps not in real time

After Effects relies on generating a RAM preview to render previews. AE is not an. NLE like Premiere Pro. It is not designed to edit videos. It is designed to create visual effects shots and short animation sequences that cannot be created in a nonlinear editor. The length of your preview depends on available system resources, comp frame size, comp frame rate, and the effects you have applied. This can be slow because AE looks at every pixel on every layer on every frame before it can create the final frame. The beauty of an NLE is that it looks at video streams and tries to play more than one of them in real-time by reducing the playback resolution and skipping a frame here and there if needed. It's a completely different way of handling video.

If you are new to AE, please spend some time in the Learn workspace (Window/Workspace/Learn) and then spend some time with the User Guide. You will save a lot of time.

This is how I run previews in my standard workflow. I routinely create complex visual effects comps and animations. Some of the more complex ones can take up to 6 seconds per frame to render at full resolution. I had one the other day that took about 4 minutes a frame to render. Trying to run a RAM preview on my 6 seconds per frame comp would take 6 X 30(fps) X 10(seconds) = 30 minutes to render before the preview starts. I work around that limitation by running what many call a Pencil Test. Start by setting the position of your elements, animating any movement, and finalizing the timing and composition. If. You are adding a bunch of effects, turn off all unnecessary effects, reducing the comp magnification ratio to 50 or 25%, with the Comp Panel resolution set to Auto and try a ram preview. For many comps, you can get this pencil test preview to render at 5 to 15 frames per second so a 10-second shot would take about 20 seconds to render before it would play back smoothly. Sometimes I even skip one or 2 frames in the Preview panel to get the pencil test to preview. When the timing and the blocking (layout and action) are correct it is time to start finalizing the effects. This is what many animators are calling the Pencil Test and the idea is more than 100 years old. When the Pencil test is OK, it is time to move on to Ink and Paint, which is turning on and fine-tuning all effects, motion blur, blend modes, depth of field, color correction, and everything else that goes into the shot and checking my hero (most critical) frames at full resolution, then I send the comp to be rendered. When the shots are all rendered I do the final editing, sound mixing, and final color correction in an NLE. With all of their resources, that's the way Pixar, Industrial Light and Magic, and every other professional effects and animation companies work. Pencil test - Ink and Paint - Render - Edit - add sound and final color grade - final render, then distribute. 

I hope this helps.