After the S7 patch, the whole Coliseum episode become really problematic to play.
The AI never act, refusing to react, even if is a npc or a enemy. The whole episode come with a heavy lag. You walk for 1 to 3 seconds and are throw to the begin of the episode, forcing you to walk all way back.
The enemies never die, because you have too much lag to hit someone, and after 5 seconds, every enemy hit drain all of your shields and life.
Because of the lag, you can "fall" inside of walls and rocks, being stuck and can't finish the episode, forcing you to leave and back to fight against the lag again.
I got this too. I thought it was just a bout of heavy lag, until I read this. I was playing with another person online. We were both experiencing the same severe lag (although not quite as severe as what you were describing), and it disappeared as soon as we were out under the stars.
I played it twice today and the second time I was spending more time fighting against the lag. I was able to get thru the coliseum portion despite lag, glitches and screwed up graphics (the last console is barely detectable because it is buried other wall). This episode needs major QA review since others with less powerful systems are being overwhelmed by the "lag monster".