The game should feel very fast and very smooth when you're playing. If it feels slow or "choppy", or it slows down when there's a lot of action on the screen, then there's room for improvement. The good news is the game offers extensive customization options that you can tweak to get the best out the game on your particular computer.
This guide will help you optimize your graphics performance. First of all, if you've changed anything, set all the menu options to their default values.
So, how do you find out if the game is running slowly in the first place? The engine automatically compensates for most variations in performance, meaning that normally you won't be able to tell whether the game is running "really fast" or simply "adequately fast", unless your system is really slow and you can just plainly see that the game is slow and jerky.
Start off with going to options/graphics/advanced and turning "show fps" on. This shows your current frames-per-second (fps) at the top-right of the screen. The higher your fps is, the faster and smoother the gameplay.
Now play through a game, and keep an eye on your fps. You shouldn't have any problems if this constantly stays above 50. If you are getting less than 35 fps, perform the following steps in sequence. At the end of each step, play a game and check your fps. If you are getting sufficiently high values, then you're done. If it's still too low, try the next step.
Step 1: Go to options/graphics/advanced and set the video mode to 640x480.
Step 2: Go to options/graphics and set "detail" to "low".
Step 3: Go to options/graphics/advanced and turn "bilinear filter" off.
Step 4: Go to options/sound and set "quality" to "low", and also turn "in-game music" off.
That's it! If the game is still running too slowly, you'll just have to buy a better computer :).