From Manual:
... a LOD (Level-Of-Detail - see Entities) system is used for keeping the frame rate high. It automatically switches to 'simpler' shapes of objects when they are far away from the camera, thus reducing the overall number of polygons drawn per frame.
As already mentioned, you need to compile the whole section level with Map Compiler and then Compare it, with your on-the-fly level.
From Manual:
Compiled scripts are started directly. Uncompiled scripts are compiled on the fly, then started. Compiling is very fast, and normally takes only a few seconds.
For level design you need a level editor like WED, because earlier or later you got in a certain level section problems like mentioned above.
Another Tip:
Search for smoothing groups