I trained as an architect, and I remember a representative of the St Pancras International project team giving us a presentation about the redevelopment of the station. Even then it seemed blindingly obvious to me that the Eurostar departure area was not only too small to accommodate any significant growth (or, as we now expect, longer wait times) - but that the physical arrangement of the station made it almost impossible to cheaply expand it.
We can expect longer wait times at customs and immigration, that much is clear. Therefore the queuing space between security and the controls has to accommodate more people.
Referring to the drawing below ("north" is to the right)... as of now it's tight in the departure lounge - you pass the departure controls and are almost immediately level with the east-west "aisle" through the departure lounge from which the 3 north-facing and 3 south-facing ramps climb up to the platforms.
You can probably make more space in the departure lounge by either losing the business premier lounge or some of the shops facing the public arcade (numbers 17a/b/c/d, 32, 33, 34, 35 etc). But I can't see a simple solution to extending the queueing space for departures.
(Arrivals might be easier... that's a very large hall at the base of the ramps which could easily be tensa-barriered up for arrival checks).