I fight going back and forth in my head on this. If we're talking a simple application with an OEM BPV installed running with manifold reference, as soon as throttle increases, the manifold vacuum decreases and the valve closes near immediately. If the valve is always held closed, the turbo can build some pressure pre-TB, but not much. If the turbo is allowed to freewheel, it can reduce exhaust restriction (minimally??) and be at a much higher rpm. When the valve closes, the turbo is spinning faster than if it had the valve closed the entire time and then could create boost faster. That's how I'm trying to reason it anyway.Being closed at idle helps spool. If it's not closed the turbo will take longer to spool since it also has to work to close the valve.
Send from another galaxy.