FWIW all the Tacoma Narrows bridges (including Galloping Gertie) were/are suspension bridges.

Some times my frame is so stiff I can barely get out of bed. This does not make me a faster rider. Which leads me to my point; The rider neuromuscular application of power through a flexing joint and bone structure is so variable as to render the frame question somewhat moot (this is likely a pretty easy bio mechanical engineering test as in establish some sort of flex measuring, use accelerometers, strai. Gauges, power meters & hit the rollers and likely has been done...controlling for the rider is tougher). Someday, somewhere, a rider will appear (on a pale horse?) with a mechanical heart and carbon fiber legs and all hell will break loose.