The accumulation of "10 seconds" on the screen sounds like a buffer overflow. How much RAM do you have in this?
Regarding the cpu usage of 100% when playing back mpeg2 files, this isn't necessarily a bad thing. Are there any problems with the video while playing back? Does it stall/skip/pixelate? If not, then it's ok for 100% usage because you want the cpu to use all it processing power while doing any task because it should finish that task sooner, such as decoding mpeg. If you are having video output problems with 100% usage, then it could be a few things that need to be considered such as:
1. Is the processor powerful enough to handle it?
2. Is the driver efficient? Is it software mpeg decoding (yes with that cpu usage!)
3. Does the board have hardware mpeg decoding? Is it turned on? (probably not with 100% cpu usage.)
For example, the Epia mb that you're also familiar with has the hardware mpeg decoding which should relieve the cpu. But a typical installation doesn't have that activated and you need the unichrome drivers for it. Maybe there's something similar with the Aopen MiniPC? I'm not familiar with that.