Noticed @alexs didn’t address why the Natwest card is instant. Contactless on many cards is offline, this means that the card issuer (Natwest in your case) gets no notification of the transaction at all until the merchant presents it for payment up to days later.
An offline card kind of kills the point of Monzo’s instant notification, running balance and undermines card freezing since the acquirer would need to check with the issuer for an accept/decline.
Yes, offline is risky as your issuer has no idea what is going on and no ability to stop it until it gets all these charges later. This is generally why, along with not having any authentication (PIN, etc), contactless is limited to £30 and banks generally (and in my pre-Monzo experience) only give offline contactless enabled cards to those with some kind of overdraft. Everything else is just eaten as the cost of having it be quicker.
45 seconds is kind of long but for the majority of merchants it’s more like 3 to 6 seconds and I hope you’ll agree in time that the short increase in time at the terminal is worth it for many of the benefits that Monzo provide.