yeah the combat system is a little different... you'll get used to it. watch youtube videos.
assuming you dont mean video lag then it's because you need to time your mouse clicks for fluid fighting - until you get the hang of it you'll be stopping and breaking your attack because your clicking too much.
don't judge the game on the first 2 chapters either but I have to say that the first level: kaer morhen is pretty poor - the game gets much better v.soon.