Like most new practices and procedures, such things seldom start without any hiccups. It would be unrealistic to expect that things go smoothly at all borders right from the start. Some border controls will be better prepared than others, but it's difficult to say at this point in time which ones will have all the equipment up and running and the personnel trained on the first day. Some smaller ports of entry might need weeks before the queues get shorter. It's a bit like the lottery at the moment.
Sorry I can't be more explicit, but that's likely to be the position on day one. I wouldn't be surprised if it takes weeks, if not months, to be working properly across the board. Airports will undoubtedly receive preferential treatment, but I'm sure that the sheer volume of traffic via Calais, for example, will put pressure that things settle down very quickly.