That all makes sense drichard. The throughput of vehicles on the M25 (UK motorway) is improved by lowering the speed limit from 70mph to 60 or even 50.
If there is an 'open run' to the portal, then people will rush in and impede each other. If there is a small barrier, then they have to slow to negotiate it and that staggers the flow and allows people at the portal to exit more smoothly. This completes the feedback loop and the frequency of people passing through the portal increases.