If your having fps problems, particularly on Ultraxion try these two fixes.
1. Turn your view distance down to the minimum. A lot of the stress from the fight is because you are fighting a boss with a ton of effects flying around whilst looking out into open terrain.
2. Look at the ground. There is no movement in this fight and as long as you can see the boss' cast bar you should never have to move your camera either. Looking at the ground pushes stuff outside your view range so it gets a lower priority when it comes to refreshing.
We had a guy playing on a laptop go from 4fps on this fight to 30fps using these two suggestions.