Gazebo /clock publish rate

edit

/clock publishes a message once per every new state, i.e. at 1/max_step_size Hz. You should rather focus on why your simulation runs so slow than the clock publish rate (did you perhaps tweak max_step_size?).

Nonetheless, if you still want to change the frequency, you can do that with the /gazebo/pub_clock_frequency parameter on the parameter server.