That vapor is a water gas. It is being produced at about 2g per second. 18 grams of water have about 30 liters (1 mol at 373K) at 100C. So, 2 grams of water fills 3 liters or 3,000 cm^2 passing through an opening with an area of 2 cm^2. So, you need to pass a cilinder of 1500 cm in 1 second to keep the pressure constant inside the boiler. That means, 15m/s.
That's fast as a hair dyer wind at maximum power.
