So my question is, has everyone else had a similar experience? I haven't seen anyone mention it on the list, so I've always thought It might be a anomaly with my configuration, but I have a new computer now, running Win 8.1 pro 64bit and it's the same situation as my 2012 macbook pro and older systems.
You're not seeing an anomaly. There's noticeable overhead to start the background processes, and while that may make sense for a long final render, it doesn't always pay for shorter previews.
After Effects CC has a new preference that allows you to keep multiprocessing on for the render queue, but turn it off for RAM preview.