RE: zAGI 5C
It does sound like a lack of control surface movement or perhaps blow back of the control surfaces if the linkages are sloppy, or the push rods are bending under load.
It will be less responsive to turn commands coming downwind; though it's ground speed may look high, it's airspeed moving with the wind may be very low. A dab of down elevator before aileron will speed it up and make it more responsive but it sounds lake a lack of control movement. These wings are usually too responsive on first flights!
Are your servos moving in the cut outs?