For UK citizens, we don’t need a Visa to travel into the Schengen Area and we can stay up to 90 days, providing we have a valid passport.
What slows us down a bit at border controls is that we now have to go through the full passport control instead of whizzing through the fast track lane for EU members.
In practice, I haven’t found this a problem at airports in Europe - they need to scan and stamp your passport - I presume that’s so they can check you’re not on a “wanted list” and to monitor your date of arrival and departure.
I have seen the Border Staff asking some people for more details about their journey and return plans etc but the majority of the people in the queue get through passport control pretty quickly.
I think anyone attempting to travel through Dover just after the UK schools break up for the Summer hols must be barmy - even when our kids were at school over 30 years ago, getting to the Dover ferries in the school holidays was a nightmare - especially in the first week after the kids broke up from school.