Not sure about flex, they're supposed to flatten out to reduce drag but I've read a lot of comments that this doesn't happen as much as they say.
For any mechanical fan, the best efficiency is the largest diameter, most blades, and turning at the slowest speed. The fan's air movement increases as the square of diameter, so even a small change helps.
HP loss goes up with the cube of RPM: 2,500 uses almost twice the power of 2,000.
For comparo: a 19" fan turning 2,000 RPM moves the same air as an 18" fan at 2,228 RPM, and uses 28% less power.
