Re:
Terrain,fitness and skill level being equal, modern will be faster. The pro argument, the better traction, control and ability to take the fastest line rather than that line minus ruts, 4inch babyheads etc. make that blindingly obvious to me.
It's that last point which clicks with what a few have already observed about retro rides; they feel faster/more characterful because it takes more attention to tackle the same terrain.
I'd frame that in terms of how involved I feel in the ride; my stage of life (3 kids 5 and under, long commutes), my release is 1 hr forest and bridleway blasts midweek. On a bike with any squish, these are tame rides; on my 96 rigid m2, every root, rock, hollow needs unweighting/movement and a bit of care to keep the pace up and have a nice flow to things. I'm not particularly quick, although hold my own with two more experienced friends on full squish, but it's the very effort/mindfulness of riding the stumpy that gives me complete absorption and escape from Life that's such a massive part of why I ride. I leave the Strava KOMs etc to others who get more of their reward their kicks from outright speed.
On the disc vs canti brakes, some of the posts are beyond parody even for a retro forum! An engineering friend, and fully subscribed retro + modern life-long rider, once explained this to me in terms of where the brakes apply their force in relation to the inertia/momentum of the wheel. This was in response to me asking why rim brakes with the biggest disc/lever possible weren't still the standard. Not having a brain built that way I didn't quite get it, but intuitively see that discs seem more capable of taking the rotating energy out of the wheel/axle without putting it into a skid. Looking at cars, I think the discs get bigger to give more area for the calipers to grip, not because of the bigger diameter/leverage at the disc rim...