use f11 mode (and the tab sliders) to turn things on/off and see what causes the biggest fps drop, then you know the area(s) to improve.
if i was making a large map in GG i would try making hills taller and/or more frequent to hide entity populated areas using a script to hide the entities as needed - though this may be a bit late for you as it's not possible to use this method for static entities...
for anyone who wishes to try it though you need to;
1. set your clutter entities to "static = no", "isinmobile = yes" and "strength = 25"
2. assign the script to an entity with "static = no", "always active = yes"
3. build up your hills to hide the populated areas
as said in the video, objects that are static or don't have 25 strength will not be included in the system - works well combined with the default GG occlusion but of course is not a magic solution, you will still need to plan your layouts.
i also recommend making sure vsync is set to 1 in setup.ini
*video being uploaded*