I just finished the Tier 3 version of this map and something is causing it to run very slow. I thought it was the connection to steam but rebooting didn't help. I also shut down anything else that might be using bandwidth and it didn't make a difference. Ending a stage would cause it to pause or go black screen briefly. Trying to finish/start missions was very slow on changing screens. Overall it just felt glitchy.

Once finished I went to T3 - Miwschief at Mugwarts - From the Ashes and it's running like normal, smooth, fast transitions between stages, etc. so I have to assume it's something with that stage in particular.