About ReadMotor instruction execute time

LIJIANHUA

I suffer a strange problem, when i use the  ReadMotor instruction  to read the position of a motor,  it need take 8-15 ms. Can anyone explain why it takes so long? And why the execute time is changing? In addiation, i test the follow four instruction  GetJointData CJointT CRobT GetMotorTorque , also find  the execute time is change from 1-5 ms. Has anyone encountered a similar problem?
RAPID code:

        ClkReset clock_state;
        ClkStart clock_state;
        !Cartesian Coordinates
        motor_position:= ReadMotor(1);
        ClkStop clock_state;
        time:=ClkRead(clock_state);
        TPWrite "time         " \Num:=time;
Thank so much!