There‘s got to be a Vertical meter threshold below which altitude is faster and above which it‘s slower, because the reduced air reaistance isn’t that helpful on climbs. I doubt 1500 meters is that much faster anyway. Overall, the average speed might be slightly higher than it would have been at sea level, but not by much.
It's easier to calculate a gradient at which lower altitude is faster, and I remember calculating from the TT that would be around 2% on a TT bike.
Now it's ofcourse not only altitude that drives up the speed, and tactics tend to matter a lot more normally. But I had not expected the u23s to beat 41kph on average.
Now, a big way WC or circuit races can slow down isn't necessarily a difficult parcours, it's simply that they can sometimes simply go suuper slow in early laps, as evidenced by 2024 Montreal, when Pogacar's fastest lap was around 46kph average, but his slowest lap was like 32kph average early when the pelotn was asleep. This is simply way more likely to happen in the men's elite race, and the most important reason the men's elite race is sometimes slower than the u23s, juniors or even women's elites.
But then, the going super slow on early laps, also tends to be much more a thing on longer climbs than shorter ones, and on technical descents and then flats than big sweeping downhills.
Final thing that really depresses the speed is when riders basically get super exhausted and lap times go way down, and that really doesn't happen in the junior and u23 races cause they're not long enough, but it happened big time in the 2023 and 2024 men's elite races.