Depending on your free ram that sounds about right. Disk cache will help if set up properly. We don't know anything about your AE version down to the decimal point or your OS, or your memory settings or cache settings. We also don't know anything about the comp. A ram preview of a comp that is using a bunch of particles and a bunch of video sources that require a ton of CPU work to decode is going to eat up resources faster than a simple text animation.
This has been an issue since I cam remember, CS4 possibly earlier, i am using the latest CC now and on a new Mac Pro i don't see why this is still a limitation.
Mentioned above I have disk Cache enabled and set to 72GB, Caching on the 1TB internal Mac Pro SSD. If I render out the clip full res it only uses 6GB of space and I have the comp set to 1/2 res. It is a simple panning background made up of about 20 PS layers, very simple, I don't know why that would make a difference, isn't Ram preview just rendering the frames out to ram? If there is movement it renders a new frame right? and there is movement in every frame. CPU is not my issue, it is the length of the timeline Ram preview will render (not how long it takes). Why can it Ram preview 30 min of footage if needed to my SSD and play it back? It seems like it would be simple!