Too many vertex format changes in frame, whole world not rendered

[Fail Spy]

I am currently on Downtown_v2_fiend_v2b and I am getting a console error. Every map I have experienced this error on gives off an incredible amount of lag. It there any fix to this? What is causing this?

Too many vertex format changes in frame, whole world not rendered
Too many vertex format changes in frame, whole world not rendered
Too many vertex format changes in frame, whole world not rendered
Too many vertex format changes in frame, whole world not rendered
Too many vertex format changes in frame, whole world not rendered

^ Thats the damn error.

[Comfy]

Its because the map is not optimized. ask the mapper to fix his crappy map.