I'm guessing you've played it on the PS3 version, Loony?

How to Beat Desert Sahagin - Lightning Returns Final Fantasy XIII Boss Guide - YouTube
I used this as a resource, and there is NO framerate drop at all for the PS3, it seems. The entire fight is sluggishly low to the point where it's IMPOSSIBLE to block unless you block 3 seconds in advance of an attack. At which point is fairly impossible because by the time you see it doing the Rush attack, you're floating in the air, and it takes forever for you to land on the ground. I wonder why there is no footage on it for the 360 version yet. It's embarrassingly bad.