Not to sound condescending, but I get this literally all the time. When you quit apps, you don't just click the red upper left icon, do you? cmd+q, or click the app name in the menu bar, then quit.
Regarding Fusion Drive, trust our fellow posters. I literally notice no difference between my Fusion iMac and my all SSD rMBP 2014. And I'm using way more than the SSD portion. You see, OS X doesn't just move files around. It moves blocks (small pieces of files.).
I was going to argue that the processor/GPU setup may be a little too slow, but if you won't do anything intensive with it, well, it's way more than enough.