I find it very hard to believe that frame flex is something beyond the realm of human observability. I feel that I can feel it and see it when I am riding. I also find it hard to believe that a heavy stiff steel frame is faster in any real way. I have tried light super stiff modern aluminum frames and they both feel faster, more responsive, and unbearably rough on the body. If frames are not vertically flexy, how come a modern aluminum frame (without a carbon fork) feels like riding a light weight concrete bike. I have ridden steel frames that have no particular "breeding", weigh a bit more than a high end bike, have similar dimensions and angles, and yes they do feel quite nice, but still certain "well bred bikes" noticeably feel tighter, sportier, more pleasant and more comfortable and somehow faster. Some of this falls in the category of subjective ramblings, but the fact that some frames glide over bumps and some seem to emphasize them isn't subjective. You can stand on the bottom bracket with one foot while you hold the bars and seat, and as you push you can see that some even very lightweight frames seem to be laterally very stiff compared to even some heavy bikes, while these same lighter bikes also seem softer on the bumps compared to the heavier models. To me, this means that a bike can be vertically compliant while still being laterally stiff. If all these qualities that high end steel bikes exhibit are imaginary, than I would still choose to be fooled into riding the "imaginarily better" high end model, even if some scientific data might indicate that I am a dupe.
Garth Libre in Miami Fl.