The image shows a flow field around a wingtip without a #winglet. More precisely, it shows the extent of positive inflow angles at the tip region. The addition of a winglet would utilize this flow field and increase performance, only if it is inside the boundary of positive inflow angles. At higher flight speeds, the positive-inflow-angle flow field becomes smaller and areas of the winglet outside would have little benefit and creates additional drag. So that is why folks, you should avoid too large winglets! Source: Johan Bosman, chief engineer at Jonkers Sailplanes.