I just encountered it the other day and a quick look at the bioware forum shows there are quite a few others experiencing it too. Cross platform as well, so it's inherent somewhere in coding, not caused by hardware or software conflicts. Basically, when you max out the friendship of a certain pirate captain you get an attack speed bonus for having her in the party. Problem is there's a glitch that turns it into a negative value and stack it so after a while your character's attack animations get slower and slower until it starts looking like something out of the matrix. Just Hawke and just the attacks mind, everything else works fine but as you can imagine, a rogue that can't move fast is dead meat.
At first I though it was because I had the difficulty on 'hard' but even after I turned it down to normal and then eventually casual, my party kept getting wiped out before I twigged it was a glitch.