Roran: They DO slow you down. Not by as great of a deal as some may think. But I have worn a set of 1600's knight armor. Made entirely after the teutonic tradition. Those things are heavy and cumbersome. You are bound to be slowever evn with muscle and training. However, you were still suprisingly agile as armor have free joints and were made to be suprisingly ergonomical. A knight in full armour could litereally wade trough peasentry and kill countless due to the sheer protection and overall effectiveness of their armor. The weakness is the weight vs terrain factor. Thats a big reason to why the frrench had trouble fighting the English. Chavalry got bogged down with bad terrain and a knight who loose his footing will have a hard time getting back up.
As for weapons versus armor:
The war picks and battle hammers were great for that. The pick could easily be slammed into armor and used to drag the knight down, provided you didnt get chopped down first. A well aimed hit to the head rendered all that armor useless with these weapons. Crossbows? They were fantastic when the Flemmish and the Swiss used them. They had massive shields that also worked like a tripod. Together with spears (Like the Flemmish pikeneers) it was a foe that Knights very much disliked going up against.¨¨
Edit: Also Armor+Momentum+Shield was a valid tactic that a knight on foot applied to many a poor infantryman, A full body tackle is like getting hit by a small truck, your head whip around and slam into things. I've see guys literally be flung aside from a shieldbash. Hell, when we tried out the armor they had me charge full body into a a guy with shield. The guy was bigger and stronger. But my 119.5 ki (90without armor) kilos+forward moment totalled his footing. This of course has the drawback of you being able to fall aswell, but you cannot have everything in the life.