AE ram preview with multiple video footage is slow

Short Answer: Adding layers increases the time it takes to render a timeline.

Long Answer: After Effects calculates every pixel on every frame. You have ten nested compositions stacked up in the timeline. They appear to be HD, and the Render Me 2 main comp appears to be at about 1920 X 1080 at 30 fps. I have no idea how many layers there are in each of the nested comps. I don't see any track mattes or effects on any of the ten layers in your main comp, but I have no idea what is happening in each of those ten comps. You could have dozens of effects and layers in each of the nested comps. The layers are also 30 seconds long. That is a very long time for a single shot. Most of your compositions should be only one shot unless there is no other way to make a transition between shots. At the very least, After Effects will have to calculate the interaction between 2,073,600 pixels for each layer. That's about 21 million calculations per frame if there is only one layer in each of your ten nested comps. You have comp resolution set to half. That is about 10.5 million calculations per frame. The render time is going to increase significantly with all of those layers. It has to; It would if you were using any compositing application, although some are more efficient and faster than After Effects for some effects, they all would slow down if you stacked ten layers on top of each other.

I routinely create composites that have 30 to 50 layers. Most layers are just small parts of the frame. Render time at full my most complex projects can climb to several seconds a frame at full resolution. When the render time climbs to more than a couple of minutes a frame, I start thinking about redesigning the comp or pre-rendering some of the nested comps. I also create a lot of comps that render several frames a second. It all depends.

If you set your Composition resolution to Auto and you turn off motion blur or use different preview settings, or even skip frames and turn off render intensive effects like particle systems, you can render what I call a Pencil Test to check the timing, blocking, and framing in your shot. Then, just like major animation houses like Pixar and Disney do, you run what used to be called Ink and Paint, you turn on all effects, motion blur and set the comp to full resolution and 100% scale, you can check a few of your critical frames to make sure you are getting the look you want and then send the comp to be rendered without ever waiting for a full-frame, full-resolution preview of your composition. A full-frame, full-resolution ram preview of a 30-second composition is going to take as long to render as it would to render the final composition. That could be hours.

Screen shot of my AE with all the footage