CO2 doesn't suffer the same cool down effects as green gas, and firing quickly won't always necessarily cause the performance to decrease any faster than if you took your time over it.
Sometimes performance can be bad right from the off just down to the ambient temperature being quite low.
CO2, generally speaking is used in order to keep GBBs operational through the colder months as green gas tends to become completely unusable, at least in my experience, between around October to around now.
Also, the problem could be that the magazine isn't inserted right. It could be fitting well enough for the striker to release gas, but not well enough for the gas outlet valve to deliver the gas into the mechanism, meaning a lot more is being wasted than would be if the gas valve and the mechanism were meshing correctly.
I'm making the assumption that the CO2 canister is fitted inside the magazine, if this is the case then just make sure there's nothing in the pistol grip that shouldn't be and that the mag is seated correctly.
If the CO2 canister doesn't fit the magazine, then you're going to have to try and give more details about the problem because I really can't think what else it could be.
