Having an elevated battery temperature could also cause the aircraft to reduce propulsion output. You can check your battery logs at healthydrones and see if you came near 60C.
Anyway, while in the middle of the mission (and if the aircraft is still within RF range of the controller), you can increase the speed of the aircraft in real time by pushing the right stick up. In your settings, you can speed it up to a max of 54kph. You can also slow it down the same way, by pulling the right stick down. It's possible that you may have inadvertently moved the right stick down on its way back, causing the aircraft to slow down.