I have a phantom 2 with the h3 3d gimbal and a fpv system. Recently I have discovered that after about 20% battery used the copter will not accelerate vertically nearly as fast as it used to like 1/2 the speed and after about 50% it barely goes up and down anymore and makes it impossible to fly. Both my batteries have 21 and 23 charge cycles on them and the copter is only 10 months old. I have contacted dji and am waiting for there response. Just updated the firmare and still the same thing. Moving the copter horizontally with the right stick it performs perfect. Has anyone had this problem before?

Have you done any of the calibrations lately? Such as the IMU and compass, they gather info from the sensors and provide the data to the main flight controller. They use thei information to stabilize the quad from all sensors including the gains and weight distribution. Might help if it's not a hardware issue.