Yeah, I would say take a look at your visleafs.

In big open areas like yours, you should make most world bushes 'func_detail', and most of yours arnt. This also will help with your compile time, and thats quite slow also (the portalflow bit)

Since the whole area is visible from nearly every angle, then all of your world brushes are making extra visleafs that arnt really being used, but will slow things down.

Making your world bushes (including the invisible ones) func_detail should help with that.

Also, that info_paint_sprayer you've got set at a speed of 75, try lowering that, as the paint can slow a map down a bit, if its already big. maybe 20-30 and see how it goes.

But, those things should probably make the whole map lag (on slower systems), I'm not sure why it would effect the map only at the end.

Is there something that you have to do, that suddenly makes it slow at the end, or is it just being in a certain spot?

Rob.