Odd delay when switching with rule

I have the following rule:

rule "Bath Switch ON"
  Item WallSwitch_SceneNumber changed to 1

It works, but with a variable delay of up to 4 or 5 seconds. It is as if the switch was updated every x seconds, and whether I see the effect quickly or slowly depends on the phase in the cycle I am when pressing the button of the switch.

But: If I switch the relay in the “Control” panel of PaperUI, the effect is instantaneous. Even if I change the scene number of the switch, there is no delay!

So it is a problem in my OpenHAB configuration. How can I make it propagate the action immediately?