My latest experience: booked on the 9am(ish) service to Brussels, ie a busy time at StP though just past the worst. I entered the Eurostar area barely more than 10 minutes before the 30-minute ticket gate deadline; the place was intimidatingly full. I asked the first staff person I saw for information on the queuing and I'm immediately directed round to a dedicated queue entrance for my service. The separate queue - to a pair of gates just for this queue - gets me in within a few minutes of getting to the station. But in the cramped space inside, with (by now) the whole of my trainload, plus what looks like a large proportion of the Paris service half an hour later (who've gone through the ticket gates more than an hour ahead of their departure), everyone is moving very slowly. After the reasonably quick baggage check (I remember that my belt never seems to trouble the machinery these days), everything is at a crawl. After a while my train then starts boarding, and I've quite a way to go - in a barely-moving queue - to the last stage of the passport checks (though I don't worry, since at that stage it's not my problem, given I don't have a time-specific onward connection from Brussels...). After a while, a guy with a little sign starts battling through the passport queues trying to round up anyone for my Brussels train; we're all (rather confusingly) walked through/across another queue, and end up at a separate manual passport booth (I went nowhere near any automatic gate that morning). I tag on to the people heading up to the platform and get to the right carriage with time to spare.
Lessons seem to be: the current St P situation with current passenger levels is just about viable; it would be more so if fewer people turned up more than an hour ahead of their departure; it relies on efficient separation of queues in the waiting area prior to the ticket gates. The melee inside would be much improved if the passport checks area had clearly designated separate queues for different trains so that, at any time, most gatelines were allocated specifically for the next departure, with allocations gradually changing to the following train as it became possible without risking the boarding of the pending departure. But to enable people to filter into labelled passport queues would require more slack space after the baggage check and before the passport checks. And this would be helped by an extra holding area at that point in the system (or by people not arriving so long before their train!).
When I returned from Brussels, on the last evening service, the Brussels check-in area, as seen from the concourse, was largely deserted (as it often is) 25 minutes or so before the ticket check-in deadline (ie 55 minutes before the train departure). Presumably because most people had already arrived and gone through the system. After a trip to a chip-purveyor, I returned to the Channel check-in area 10 minutes later to find there were now as many as a few dozen people ahead of me in the system. From walking into the "Channel terminal" from the main station concourse, navigating the ticket gates, baggage check, both passport checks, and the awful duty-free zone, to getting to the seats in the waiting area, took about 7 minutes for the lot. About a minute of that was spent zig-zagging up and down between empty queuing barriers, and another minute or two was wasted because, for the first time in ages, my belt triggered the scanning gates - so I lost time being ticked off by the security and by subsequently needing, in an undignified fashion, to try to put my belt on with one hand while holding up my trousers with the other hand!