Thank you Bobsofia. I plan a trip exactly in early August :)
This question is inspired by this photo taken july 2017:
As we can see, queues are quite large, but maybe it is not a rule...
As a EU citizen you shouldn't have problems with passing through. It's just the larger summer traffic as @bobsofia mentioned that could lose you some time. Sadly there is no proper way to avoid it. Last time I was at Kalotina in the summer peak was in 2016 and it took roughly 1 hour to pass through all the way.