My fibre glass fin gives no more propulsion than, for example, the Rapid or any kind of bi-fins I used with dolphinkick.
Granted, I remember trying one competitor which seemed to me at least 3 times faster than any other fin I have, but that was a long time ago and there is no way to verify this with a timed test now.
It probably also depends on the swimming style, but for me personally, I find swimming with not so stiff fins much more comfortable- and not slower in the long run.
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