Uhm, turn of the BG Effect?
you answered your own question.
Create your own Colosseum map.
Look into the BG Effect why it's creating the lag. Maybe you are missing something, maybe the effect is too big, is it your own BG Effect? or do you just copied it?
Is it really the BG Effect? or do you also turn something else off when you turn off the BG Effect.
It can be so much...