Why do I have to hold the phone against the lock for a few seconds?

The normal waiting time (3-4 sec) is because the lock needs power to check the access rights and to open. That power is drawn from the phone through NFC. The technology only allows a little amount of power (50mW) to pass through NFC so it takes a couple seconds for the lock to wake up and function. It is a is a small price to pay for stability and security because the lock does not need wiring or a battery to function and thus can not run out of power.