At what speed does VFR become too difficult?
Assuming one is flying around the packed airspace of Southern England at low level, trying to get words in edge ways, dodging this and that while trying to stay aware of your position with nothing but DR and the map.
At what speed does this simply become too difficult? At 100kts it's possible, but at 250kts say would it still be practical or would it just be a matter of time before you ended up busting airspace?