I had plenty of lag, apart from all the error 37 issues about which I was complaining in another thread already
. Sometimes I have to log on multiple times or quit entirely, because the lag is so bad it's unplayable. Especially now that I'm on nightmare, where I already encountered the first couple minibosses with ridiculous skill-combinations that can stun/fear and then smash my barbarian in a couple of hits if I'm not careful. One particularly annoying skeleton cast fear on me almost constantly, had extra health and healing. Without my own stuns I would have never been able to smash him to dust, unless the companion AI would have suddenly become competent (very unlikely).
On normal there is no difficulty curve to speak of. However, there are some brick walls that I hit where the game just threw so many enemies at me that at some point my health-pool was not large enough. But the only boss who gave me trouble and needed one than more try was Belial. But that was probably because my equipment was rubbish most of the times. For large parts I ran around with white (non-magical) weapons. Once the auction house worked I could buy better gear, now I'm feeling I have halfway decent stuff.