The backpressure increases the trapped mass in the cylinder (and is where any motor will see reversion) but the port velocity is consistent for most of the duration of the intake valve being open when the exhaust valve is closed. The 2:1 pressure ratio across the engine you're seeing isn't reducing the port velocity because the pressure differential isn't 2:1 anymore once the exhaust valve closes and the cylinder volume is changing as the piston is traveling down from TDC.