For information, the OnClick event is sent when the pressure is relieved, that is, when the finger is lifted, like a mouse click.
If you want an event to be sent as soon as the button is touched, you may require PointerDown which can be managed through the Event Trigger component. it's one rare component that cannot be deactivated, so you need to combine it with a Canvas Group component and control the Block Raycasts boolean and set it to false to neutralize the Event Trigger (it ignores the Interactable flag).
Or, instead, you can use Playmaker's UI POINTER DOWN.