First of all, when you post code, Please use code tags: https://discussions.unity.com/t/481379
Without seeing any more, I speculate that the delay is within the PauseMenuCoroutine() coroutine.
Set the button’s .interactable property to false and it will stop responding to clicks.
https://docs.unity3d.com/2018.3/Documentation/ScriptReference/UI.Selectable-interactable.html