what is the real unit of roation speed of dynamixel MX-64 in jointPositionMode

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Hi guys, I'm using a dynamixel MX-64 with a hokuyo laser mounted on it. the motor works under joint position control mode, and I set it speed to 0.25. As my understanding the unit of speed is radians per second, as using dynamixel control, but it turns out that when It moves 1 rad, takes around 2 s, which should be 4s instead.... someone may say it no-load speed, however I run it with load, it should be slower rather than faster, right? also this fact doesn't match the unit RPM of dynamixel manual .

So what is the exactly unit of this speed para in dynamixel driver ROS package ? Any help ?