optimisation

I'm pretty satisfied whit the layout of the map, but I want to optimize it before artpassing it. I have already done some minor optimization in the form of area portals but they don't really do the trick for all the places, can someone help?

I'm generally hesitant to write about topics that others have already covered. But it seems that I continually see maps get the same things wrong, especially in regard to optimisation. So this guide will aim to provide a wide breadth of knowledge about intermediate-advanced level optimisation...