Nothing wrong - if you merely play a simple footage, there is nothing too cache and AE will try to use fast preview. The cache only matters, if you apply effects or do things to your footage, not just review it.
I tried turning off all the layers except the video layer, hit the space bar, and got a non-realtime preview of 15 - 18 fps. There are no effects whatsoever applied to the video. I've used AE for years and have never seen realtime playback without doing a RAM Preview, so that's not my expectation. My issue is that AE does not seem to be creating or storing a global cache.
OK, here's what I've found: On my workstation as well as our test system (now set up with a dedicated RAID just for the cache), the GPC seems to work when the resolution is set to 1/2 rather than Full (before changing it, on my workstation the resolution was set to Auto and it chose Full).
Is there a way to get GPC working when the resolution is set to Full?