YAW rate very slow in 3.4.4

Just upgraded my F450 from APM 2.6 to Pixhawk (lite). Installed 3.4.4.
Amazing software, except one thing:
The yaw rate now is very slow: It needs almost 3-4 seconds to turn 360° with full rudder ! (1900 or 1100 pwm) (tested in Stabilize and AltHold). Pitch and roll are pretty responsive.
Radio calibration in MissionPlanner shows that 1900 pwm are recognized.
I have set ACRO_YAW_P from 4.5 to 10 - no visible change
Tried 100-400 for MOT_YAW_HEADROOM - no visible change
ACRO_Y_EXPO is 0.2

With APM 2.6 and arducopter 3.2.1 i could have 360° turns in less than a second.

Can anyone help me ?
mypix3.txt (10.1 KB)

OK, found the solution myself: ATC_RATE_FF_ENAB has to be set to 1