Hm, it doesn´t look as if the transition is the problem at the moment, but the engines light manager. Try to set max_lights higher, make sure that you build in mesh mode and check the center of the levelparts and the lightrange. And try to turn off the light maybe. You should may also think about using shadowmapping for those lights, at least if the speed of everything else does well.
And you just have to think about switching to Lite-C. It makes especially the shaderwork but also everything else a lot easier. At least if you don´t already have a big Codebase where you can use big parts from, or some other important reason I may don´t understand

And I actually don´t like that shader from the wiki, especially in terms of speed...