It may be the very stout frame. I used to scoff at “planing” but then got a geometrical clone of a 2003 custom Riv Road built with narrower gauge, thinner wall tubing (I got the replacement frame because I wanted certain new features, not because of the tubing, but I asked for normal gauge, thinner wall 531 at the same time), and I immediately noticed that it felt “easier to pedal.” Now, no science at all, at all, at all went into my judgment, but I don’t feel that the newer frame is “fighting me” the way I felt over the years that the 2003 frame was slightly “resisting my pedaling.
It has been 15 years since I owned my first-gen Sam, and if that model had had clearances for true 50s and fenders I would have kept it for dirt road riding, but it did not feel “spritely” on pavement, to say the least.
OTOH, “they” have long said said that the biggest change you can make to the feel of your bike is to change the tires ….
… I’d like the bike to feel quicker and more inspiring with this wheelset, I've often thought that the "ploddy" feeling my Sam has is down to the uninspiring wheelset.