Regarding the progressive lag with the freeze command, it seems to happen in general with anything that freezes the enemy AI (like Lithium's inventory) and it happens even if the enemy wasn't alerted by the player (my theory is that the enemy's code, when frozen, enters a loop that consumes more and more resources or something like that).
I got the same lag problem you mentioned, especially when entering E1M10 (Sewer) in Doom WadFusion Xtras, and many other maps with more than 100+ Monsters pre-placed within. Gallagher should fix the performance issues, and I agree with people who have faced the same problem as we did recently.
How many different and unique monsters did you pack in the Mega Monster Mix? Did you include Matt's Monster Pack from ModDB? If so, thanks for using my monsters from there. You're welcome to use my monsters anytime.
Be sure to add MadCat to the credits for using his monsters as well.