As larger port flares allow velocity to be lifted without turbulence, port compression becomes an issue
Port flares allow a controlled change in velocity as air exits a port.
This can prevent turbulence in the boundary layer, close to the port wall, from becoming audible.
As the airspeed is lifted a little further, turbulence within the straight section
of the port becomes an issue.
The reason a ported speaker works is because the air in the port causes a 180 degree phase shift in the air behind the driver. When the air comes out of the port, at least at the tuning frequency, it is in-phase with the front of the driver, giving a boost to the output.
As air in the core of the port becomes turbulent, this phase shift decreases, thus reducing the in-phase contribution made by the port. The total output of the box no longer rises at the same rate as input power is increased, and the port is said to be compressing
Eventually the port acts as a mere hole in the box, short circuiting the acoustic output of the driver and allowing excursion to become uncontrolled.
The compression of an unflared port worsens in a steady curve. A flared port "hangs on" for longer, and then quickly plummets. Ultimately both ports fail at the same level which represents completely turbulent flow throughout the port. Much like a car racer using slick tyres, the trick is to use the extended capability of a flared port whilst making sure it's not pushed to the point where it "lets go"
Factors which come into play include air velocity, port diameter, port length and port flare radius.
Actual figures for the amount of compression are beyond this short analysis, but the flare-it calculator can indicate the velocity at which compression begins to compromise performance.