dddd

Seems to me that a stiffer frame better allows the muscles to meet resistance throughout the useful range of leg extension, which should improve muscle output.

But of course the stiffness difference between various normal frames only shows up as better motion control when leg force is at a maximum, so makes little or no difference when riding at a sustainable output, and with less-forceful riders experiencing less effect from a frame's flexibility.

A frame with too much flex though will mimic longer cranks from the leg's point of view, which might lead to joint soreness.

The modern bikes do have a few tricks up their sleeves, as has been mentoned, but I used my '71 Super Sport today with Twin-Stick shifters on a fast-paced 51-mile ride up into the foothills, rode hard and only dropped back toward the tops of several extended climbs, this obviously due mainly to the bike's 30+ pound weight.
I blew a 27x1-1/4" back tire out thru the middle of the tread on the very last climb of the ride, older Cyclepro skinwall tire, but booted nicely with a once-folded $ bill and was even able to patch the jagged hole in the tube. Got back in plenty of time for beers/dogs and socializing behind the local shop.