Odd delay when switching with rule

Setup, Configuration and Use Scripts & Rules
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1

I have the following rule:

rule "Bath Switch ON"
when
  Item WallSwitch_SceneNumber changed to 1
then
  RelayBath_Switch.sendCommand(ON)
  WallSwitch_SceneNumber.postUpdate(0)
end

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?