If the speed is decreasing due to the battery voltage dropping under load, you would need to use a battery with a higher output to reduce the voltage drop.
But I suspect it might just be that the bike is slowing down until it matches the maximum speed at which the motors can actually climb the hill, as the bike is unlikely to go up hill anywhere near as fast as it does on the flat.
If the incline is consistent and you were to start climbing it from a slow speed, the bike should accelerate until it reaches its maximum climbing speed and then it should maintain that speed for the remainder of the hill (unless either the battery voltage drops or your pedal assistance decreases).
If you started the incline at a higher speed, the momentum of the bike, rider and battery etc. will help to maintain a slightly higher speed until the bike eventually slows down to the maximum climbing speed that the available motor power will allow.
It's just like on a conventional bike when you pedal as fast as you can to take a run up to a steep hill, when you hit the bottom of the hill you are moving pretty fast, but the bike will inevitably slow down even though you stand up and start to pedal much harder!
Alan
