I'll see if I can put this in English. Others may disagree or have variations, but this is my understanding. There are, very basically, two things which slow the game or FPS down. Polygons and scripts.
Polygons are what the engine must render. Everything in the game world you make is made up of them. The engine must continually draw these polygons on the computer screen. The more complex these polygons are, the longer it takes to draw them, and this can slow the game.
I have to explain something here which will help make sense of this process. You've probably heard it said that large open rooms will slow the game down. That's because there is a large number of polygons in view, or being drawn by the engine. As your character turns around, each polygon in view must be quickly redrawn many times to give that illusion of you turning. When we make small rooms, short halls, and many twists and turns in our map, we cut down on the number of polygons that can be seen, and therefore drawn by the engine.
Keep in mind that the engine looks right through windows and even doors, drawing what is on the other side. Keeping them to a minimum and staggering them helps to keep the field of view down.
To me, the larger draw on FPS is scripts. When objects are static, they all become part of the big picture and all the polygons which make them up are fixed. When objects are dynamic, they are assigned scripts which allow them to perform as we want them to in the game, to make them interactive.
When a dynamic object has a script, the engine must not only draw that objects polygons, but must continuously read each line of the objects assigned script, checking to see if any of the scripts conditions have been met.
Imagine, in this example, that the computer can read the lines of a given script in one hundredth of a second. Now imagine that you have one hundred dynamic objects running scripts. It now takes the engine one second to read all the scripts. That sounds pretty fast, but it has to be done continuously, and while you're trying to run and gun through the level. The engine does everything progressively. It can't draw the polygons that create the illusion of movement while reading a script. It does one, and then the other.
That's my take on the problem - polygons and scripts. Keep them to a minimum to keep the FPS to maximum. There are many tips and tricks for doing this in the Official Guide. Look it up in the stickies, and since I don't remember seeing you post before, Welcome to the forum.
Best.
I know.... way too much info.
