On the commuter path to work the retro bike might be faster. I never stopped riding nor buying new bikes as technology updated and I can tell you that I could not do the things I do now on an equivalent bike from 20 years ago. No way. I could ride up and down the same things, and potentially the up part would be about the same, but the thing would self destruct at the pace you can descend now. IMO bikes have never stopped getting better, although from probably '97-2003 everything was a bit 'experimental' as technology changed rapidly and reliability did not keep pace.
Now, I ride up to get to the down. If I were out for an all day fireroad marathon, I could probably do it just as well on an old rigid bike (not an old suspension bike as something would be irreparably damaged by day's end, no doubt).
Heavier...I don't know, my 'All Mountain' 29er has big and sticky-soft 2.35 Schwalbe Hans Dampfs and wide Stans Flow EX rims, 130/120mm suspension, 780mm wide bars, 4 piston Zee brakes and a dropper post, its 30lb. My '96 LTS-2 is only about a pound lighter, but its got Q21Rs and M56X LX, no fork lockout, 1.95 tyres, no chain retention etc.
I took the LTS out once on the same trails and had blown the rear shock about 20 minutes into a 1hr descent/traverse trail, all whilst struggling to maintain pace with guys I usually drop because it simply didn't stop, the suspension didn't really work and it had no grip. For two middle of the road all-purpose full suspension bikes, the comparison was night and day different.
You adapt your style to suit the bike, yes, but adapting it to an old bike just means cutting the speed back and taking all the B-lines.
FWIW I don't tend to use the rear lockout except on tarmac and despite the LTS being efficient by 90s standards my 1lb heavier big wheel bike runs away from it on climbs, as did my previous similar design modern 26" bike.
There are crap modern bikes, brilliant older bikes, and applications which will suit any of them perhaps better than some others. But all in all, bikes never stopped getting better at doing what we do, although SOME got a LOT better at doing LESS things. It may also sound like I'm concerned about going fast but in reality, going places faster, easier, more reliably is pretty much the measure of a better bike (Not necessarily a more engaging experience).
