I disagree with that drawing on several points. Mainly because you are never going to get a string to fit perfectly in a semicircular slot like that. The string will either be larger, in which case it sits higher up on two points, or smaller, when it sits on the bottom of the slot, very much like it was a square slot.

If the string did sit perfectly in a semicircular slot, then it would have far more friction than a square slot (for the same reason that wide car tyres have more grip and higher rolling resistance compared to skinny tyres).

I’d say the V slot is the worst of the lot, followed by the semi-circular slot. First one, the sides are slightly tall, but not too much if as shown, and the second one I’d like the sides to be very slightly talker as a minimum.

It’s all very well people drawing these idealised slots, but in reality the sides will almost always be a bit wider than the string, and the base is unlikely to be perfectly flat or round bur a bit rough and random.