cant find anything in the manual....but it's my impression that you move slower with heavy armor...can this be? would be kinda logical

depletes stamina faster

Supposedly it does but i can't say i've really noticed and i'm running around with a mixture of Dragonbone, Ebony and Orcish atm. Could just be that i've gotten used to it so the movement seems normal.

hmm i think i have to try it cause the reason im usnig light armor is -besides sneaking- to move faster...dodging melee attacks is pretty easy, especially two handed...guess have to try the heavier stuff

Yes.

SPOILER: (kinda...)

The Steed Guarding Stone removes that penalty and also lets you carry more.

Yes. When in the Companions' I was running beside a heavy-armored shield-brother, and I was notably faster than him in my light leather armor. (He was chasing a deer, I was chasing after him.)