Some things we do in our developments:
* Spawn character AI only when required. For example, if the player is outdoors or indoors, we spawn the characters via trigger when the player is within a certain area (making sure they don't just spawn in front of the player of course). This way they don't need to be running in the game until actually needed.
* Spawn items the player needs to collect or items that have scripts that the player will "interact" with. This seems to help keep the flow consistent. My rule of thumb is if the player does not need it right away, then why have it appear off the start.
* Break up an outdoor level into two or three smaller levels. What this means is simply this. I have an outdoor level where the player "enters" the town. Part way through they reach a section that has the win zone and then the second part of the town will load. In the second level, I construct part of the first level town section the player had just passed by. Since there have been issues with outdoor levels, this method had helped us greatly.
* Lighting is an issue, as is settings in the setup.ini. Lightmap quality and lightmaptexsize to us are the two key things that also help. We keep our quality anywhere from 16-30, and the texsize at 256. Of course opinions will vary on this. The settings I've listed are what we use, we like them, and we stick with them.
* Although some claim to use a mod, we never really seen much in the way of increased fps performance, particularly in the levels we have designed. I'm not saying mods are not good, I am saying the mod is a tool to enhance your game development. Mods contain those "bonus" features you want to add to your development. Some say mods have increased their speed, and that is great. It did not for us, but that is us. If you try a mod and it helps you, that is a good thing.
* Video cards do make a difference. I can take a level off my desktop and run on my laptop. The only difference is the video- the desktop has a 1 GB ATI Radeon HD 4650 video card whereas the laptop has an integrated AMD Radeon 6550. I can get on average 30-50 FPS on the desktop on an outdoor level (some sections increase), and the laptop remains at 15-20 FPS until I enter a segment room then it increases. On our desktop, we had the integrated video and the FPS was the same as the laptop until we bought the ATI card.
A key thing to remember is that everyone has a different system, and some of us know our systems inside-out. That is not meant as an insult, just a fact. We have a good high quality desktop system and what works for me may not work for the person with a newer system. It is how you understand the engine and make it work for you rather than fight against it that is the key. It takes time and practice, trial and error. We have an entire notebook filled with notes on what works, what does not.

Twitter: @NFoxMedia