Pretty much to echo what was said above. If the conditions that we sail in were completly static, then yes, a perfectly rigid mast with a perfectly rigid rig would be fastest. i.e. perfectly steady wind on perfectly flat water sailing only on one point of sail.

However, since we sail in a constantly changing environment, the flexible mast (along with all the other controls) allows us to vary the foil shape to suit the conditions. Additionally, a dynamic rig which self-adjusts can be made to dampen the ride. When a gust hits, the top of the mast flexes allowing the top of the sail to open automatically. The boat stays flat and keeps going fast.

Of course, this is great in theory. It all depends on the designer to make it work. If the mast doesn't match the boat and sail, then you're out of luck.

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