When I departed from Brussels in June, it took me a full hour to get through security and passports, meaning that I only cleared British immigration at the time the train was due to depart. As I was by no means the last in the queue, it was delayed. I travel on an EU passport, but there was no segregation of the queue into EU/non-EU until you reached the head of it. I am am not sure, but I don't think non-EU passport holders were being scanned. I believe part of the problem was that the preceding train was late, only departing 45 minutes before mine, so I assume they only started processing my train once that had gone. Being stuck in a very slow moving queue for an hour in stifling heat was not pleasant.
This was a far worse departure experience than I have had at any airport in the past five years. While I would rather travel by train to Europe, it is now more expensive, slower, and a less pleasant overall experience than flying. Eurostar really do need to up their game.