Now, the <updateRate> tag on physics and rendering can use negative numbers.

When using positive numbers an limiting the number of updates, one must be careful because if the numbers are not related between them and small, some weird time effects can be seen. Also, we may just want to render a number of times each physics update without worrying about the absolute number.
This is what negative numbers do.
For instance <updateRate>-3</updateRate> in rendering will cause the simulator to update 3 times physics before updating rendering. If physics updating is unbound (as it is now) it will cause the physics update 3 times, render 1 , update 3, render 1, etc.
I have seen this behavior helps my machines here so I have made the 3:1 physics:rendering updating rate the default now. To return to old Gazebo behavior, specify <updateRate>0</updateRate> in render.
The new behavior can have problems with some hardware setups, please report if your simulations work better or worse. If possible play a little bit with the numbers, maybe even more physics updating per each rendering cycle is welcomed.

Jordi Polo Carres
NLP laboratory - NAIST