There are many options to increase fps when you get into it. Obviously we do need occlusion to be operational, billboarding or quads for distant objects as well. Both of those should speed things up a lot, but obviously so far in GG that hasn't happened as yet. i also thing improved shaders will help on the high end side but I am only guessing, as I am not up to date with more recent shader developments. I think GG uses shader v2 effects still for all effects? As I say I am unsure.
However, as we do not have this yet, here's a few tips for speed.
AI as said, will slow your scene. Try to not go too mad with it. Spawn enemies when close rather than all the time. I think this has been covered.
This could be improved by the devs by possibly with multi-core support. Most of us will have at least one cpu sitting practically idle while playing GG games. I have 3, some potentially 7 (or more, but I doubt many would be using top server kit, if only I were rich, lol), and it would be nice for some of the cpu load to be offset on those. Of course I know this is not as simple as it seems, but AI and physics I think should be multi core. There's no excuse these days for having an issue with a lot of AI. I would certainly like to see at least 50 or more in one scene running no problem if I wanted it. Other games manage thousands.
Entites. Follow a similar principle to the AI as far as not having too many in one area. Spreading the entities more always gives a boost (up to a point). Be as liberal as you can with trees, particularly animated ones. They hit performance considerably more than most other objects.
Settings. Don't forget you can lower the quality down. I know no one really wants to do this, but eventually (unless GG gets a major speed update) you will have to look at this. I have found that the terrain settings give me most fps boost by lowering, followed by entity quality and lastly grass (although quality for me makes little difference.). You will find a boost if you lower the density and distance of the grass also.
Lower the camera distance and the terrain size down. Unless you are high up in a scene you can normally have some quite low settings without seeing a jot of difference.
If your scene is all indoors, switch off terrain, you will get a massive boost. Also water can slow things a tad as well, so turning that off as well if not needed will help. In my commando game I used scripts to disable enable as needed to help improve speed across the level.
There are more options but I don't want to make this an unofficial tutorial, lol. Unfortunately, there is no magic button for speed, you have to work at it to get any game of decent size running well. TGC can certainly improve things here, but we as ever should strive to do whatever we can on our end to keep things as smooth as possible. As with all things, the more you play about, the more you will learn, the better your games will become.
SPECS: Q6600 CPU. Nvidia 660GTX. 8 Gig Memory. Win 7.