Stock, you need to find a balance between operating pressure and dwell. Within certain limits (no point using an *excessively* long dwell, as the DM shoots from a fixed volume of air, so the firing chamber would be empty before the dwell ran out) both operating pressure and dwell will effect velocity, and somewhere between the extremes of long dwell and low pressure, and short dwell and high pressure, you should find an optimum balance for maximum efficiency. Every marker is a little different, so unfortunately there are no magic numbers I can give you - you need to do a little experimenting with your own marker.
Other things to consider are lighter paintballs (less air required to accelerate them up to speed), good paint to barrel fit, barrels with minimum porting etc.
Some people have good luck with volume restrictors (top hat mod) to reduce the amount of gas stored in the firing chamber for each shot, and using higher operating pressure to maintain velocity - other people hate them.
Some people also have good luck with higher flowing bolts (orange for instance) or complete bolt kits (Evolve), but results can be hit and miss, and quality control of some aftermarket parts is questionable at best.
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