This does seem suspiciously slow for the machine you describe depending on the resolution and frame rate of the Composition, assuming you’re attempting to RAM preview full resolution without skipping frames and that you’re working 1920×1080 and not 4k resolution.
Motion blur may be the culprit as it is computationally expensive, I generally only turn it on to check the look and then turn it off as I’m working, then make sure its on for renders. It can severely slow down workflow.
You should look to see where the bottleneck is, see what’s happening with RAM, disk activity, and processor activity during the RAM preview to isolate the cause of the slowdown.