Planar move plugin make the manipulator's motion slow

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Hi, I'm trying to simulate Kuka youbot with gazebo.
I have a question about gazebo planar move plugin.
Since kuka youbot has mechanum wheels, it needs to use gazebo planar move plugin.
So I followed this instruction and added the plugin.

As a result, the mobile platform works fine, however the manipulator on the platform looks weird.
Manipulator falls down because of gravity, but the motion is very slow.
And each joint requires more torques to move than without the plugin.

Why planar move plugin affects robot motion in the above way ?
How can I avoid this situation ?

Thanks in advance.