Yes the Diana 100 is (was)designed to be a 10 metre rifle. MV is in the region of 580 fps. Not designed to produce your normal 12ft lbs, because as has been stated earlier, wadcutter 177's like to travel at that speed best.
The reason could be their highly un-aerodynamic front, which is primarily intended to punch clean holes on paper targets, this might cause stability problems if the velocity is increased too much. These problems might not be so evident for general shooting situations(+or- a few millimetres), but for competitive 10 metre shooting, it makes a world of difference.
I stand corrected but I think that the reason that the velocity is kept around the 600 fps mark is because it is the best compromise between a reasonable pellet exit time from typically a 10 to 13 inch barrel, and low speed stability dictated by its shape.