Need help ! Feed Rate Doesn't Remain constant in Siemens 840D SL 5-axis

Hi All,

i am trying to machine a workpiece with 5 Axis machining using fusion cam with siemens 840 d sl controller. No issue for 3 axis, 3+2 BUT in 5 axis feedrate does not remain constant at 1440 mm/min set in Fusion CAM. Actual reading of feedrate during machining ranging from 300 ~ 500 mm/min.

Guys, need help on how to keep my feedrate constant. I attach Gcode file and postprocessor for reference.
Thanks in advance.
Reezal

G-code sample:-
; %_N_MULTIAXISCURRENT_MPF
N10 ; T="6MM BALL ENDMILL" D=6 CR=3 - ball end mill
N11 WORKPIECE(,,,"BOX",112,0,-95.44,80,31.5,31.5,-31.5,-31.5)
N12 G90 G94
N13 G71
N14 G64
N15 G17
N16 G0 SUPA Z-0.1 D0
N17 CYCLE800(1,"",0,27,0,0,0,0,0,0,0,0,0,0)

N18 ; Multi-Axis Contour1
N19 T="6MM BALL ENDMILL" D1
N20 M6
N21 S10000 M3
N22 G54
N23 TRAORI
N24 G0 X25.216 Y-5.186 Z15 A3=0.984162 B3=-0.177271 C3=0
N25 M8
N26 G0 X25.216 Y-5.186 Z15 A3=0.984162 B3=-0.177271 C3=0
N27 G0 X25.216 Y-5.186 Z-26.223 A3=0.984162 B3=-0.177271 C3=0
N28 G1 X25.202 Y-5.184 Z-26.357 A3=0.984162 B3=-0.177271 C3=0 F1440
N29 G1 X25.158 Y-5.176 Z-26.483 A3=0.984162 B3=-0.177271 C3=0
N30 G1 X25.087 Y-5.163 Z-26.597 A3=0.984162 B3=-0.177271 C3=0
.................................................. .....................
.................................................. .....................