I just bought a new mountain bike (Orbea MX 30) and had my first ride today.
It's really nice, riding is smooth, there's a shitload of gears and it's overall better than my old beater bike but I'm a freak speed.

When I was 14 and a 50 kg DYEL I had a cross bike and managed to reach max speed of 40 km/h on flat road.
I expected that I could beat it on this new bike now that I'm /fit/ but only barely managed 38 km/h.

Is this normal? On one hand I have muscles now, including strong legs, but on the other I'm 84 kg compared to 50 kg then, even though I'm lean. Also the bike is heavier and has MTB tires, and I think it impacts how fast it can accelerate (on a longer road I could probably beat the record).

I've heard you can buy road tires for your MTB, would that increase my max speed? How fast can you change the tires once you learn them, is it a hassle?