Frame flex is a somewhat interesting discussion. Somewhat in that it depends who you are and what are your goals and objectives. When I purchased my first track bike, I met with a friend who is a 4 time and current world sprint champion. He said any frame / wheelset performs well at 300 watts in the straight away but standing in the turns at 1500 watts or over 2HP, it is hard to hold a flexible frame in the pole lane.

In a 500 meter time trial, we practice the standing start. From zero speed, we stand and lunge forward pushing down and pulling up as hard as possible. The most important pedal stroke is the first one. Each subsequent pedal stroke is less important - the start is everything.

Sprinters pride themselves on their ability to gap a competitor. They do this with with the first couple of pedal strokes. A sprinter starts his sprint from 200 meters and generally there are 20 pedal strokes to the finish. So the first 2 to 4 pedal strokes represent 10 to 20% of this race.

A flexible frame that gives back the power it absorbs on the rebound is not of interest to trackies and sprinters. Our race can be over if we miss a pedal stroke.

Flexible frames that move laterally are, IMO, not great to ride independent on what power loss or no loss theory one wants to embrace. Vertically, compliant frames with dampening can provide a great ride and I find that an asset.