When the wither moves into the second phase of the fight (turns blue and starts dropping Wither Skeletons) the lag instantly goes from 60fps to between 0 and 2fps.
Game is rendered completely unplayable.
Up until that point there is no lag.
Attempting to access my inventory during this part of the fight took about 5 seconds for it to even open.

The fight took place in an obsidian room 32 x 32 x 10 at bedrock level.
There are no builds or farms within 500 blocks of where the fight took place.
No redstone of machines built to help the fight. Hand to hand combat.