Gazebo-7 is slower than gazebo-2

I run the same Baxter robot simulation in gazebo-2 with ros-indigo and gazebo-7 with ros-kinetic. But the simulation in gazebo-7 is much slower than gazebo-2. In gazebo-2 if I set the real time update rate to 10000, it can reach around 5000, but in gazebo-7 it can only reach 2000. So the simulation in gazebo-7 is inefficient. Is this normal? And how should I speed up it in gazebo-7? Changing max step size too much is not a good idea. it will influence the simulation. Many thanks.