Being a 4 Liter 4 cycle engine, every time the engine rotates once, it ingests 2 liters of air. That's at 100% Volumetric Efficiency.
At idle and low cruise VE's pretty close to 100%, at 2500 it may be down to 80% with no restriction in the intake. That's just the restrictions from the valving, intake ducting and exhaust flow.
That's 2 Liters for each rotation.
At 2500 RPM, a normal high cruise speed, it's ingesting 5000 Liters each minute (2L X 2500Revs/Minute = 5000.)
That's an awful lot of air, or anything, to try to get through that small snout.
ANY restriction of that air cuts down the VE == power, mileage etc.
Now think when we rev it hard - 4000 RPM that's 8000 Liters! A built-in governor!!
And then at 6000 - ???
But - the issue at hand is making sure that air is CLEAN - if that air's dirty, VE becomes irrelevant as the engine won't last long.
The search for super clean and little or no restriction is the task - and put it in a small space so we can have our other goodies (OBA etc.)