The NXT is already equipped with a high efficiency bolt kit that should be giving you something in the order of 1400 shots per fill (assuming a cold filled, full 68/4500).
You said that you had experimented with dwell, but how about operating pressure? You need to find the best balance between dwell and operating pressure. Takes a bit of time unfortunately.
Another thing to consider is the condition of your o-rings. May be worth getting a repair kit and totally overhauling the bolt kit.
There are aftermarket bolts and bolt kits available, as well as firing cans and other parts, but a lot of them are more concerned with reducing operating pressure rather than increasing efficiency. The Nox bolt might help a bit, but don't expect too much from it. The Eigen bolt hasn't been released yet, but testing appears to have shown positive results.
Another thing to remember is that a good bore to barrel fit will help with efficiency, as will barrel design (a lot of porting along a long section of the barrel, for example, will waste air).
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