or just raise the fps cap.
let me get this straight, you have df running and task manager lists it as not responding? put both windows next to each other and see if df is active and still marked as not responding.
I have noticed this happening too, I think its just an artifact of the way dwarf fort was made, possibly combined with hyperthreading.
As for the cores issue, I only have 2 cores due to my hyperthreading. Basicly nothing actually has support for using multiple hyperthreaded cores though, it was a transitional technology that didn't exist very long before actual physical multicore came along and obseleted it. So most of the time, any given process on my system will only use 50% of my CPU, using one of the fake cores. Good for recovering from programs locking up, which is really the only reason I have it on.
However, if you go look at the performance tab in taskmanager, the CPU usage appears to be spread identically over the two cores when a programs affinity is set to both (as is the default for any new running process). If you switch the affinity on the process to one or other of the cores, the graph for that core then maxes out as you would expect. I think its just a case that not even windows actually knows what to do with the hyperthreading, so things that are single threaded but with affinity set to both cores, just show as using both cores half as much.