This is something you should have checked before you got the game in the first place, but if you're experiencing crappy performance, this may be something worth looking into - is your computer strong enough to run the game?
Once you know what your computer has under the hood, you need to compare that to what is required to run the game and expansions you want to use.
Your computer should not only meet the minimum requirements, but exceed them! Make sure to note all the recommendations on that page by MTS staff, indicating what will not only run the game, but run it well!
If your computer does not meet or exceed requirements, you'll probably need to upgrade - this means either buying new components to spiff up your current computer, or, depending on your system, purchasing a whole new computer.
If your computer isn't brand new, you might need to do some general maintenance to keep it running its best. Even if you're not a "computer person" it is definitely worth learning basic computer tune-up skills... not just for your game performance, but in general!
Sometimes, especially if you've recently patched/updated your game, you may experience decreased performance. This can be due to outdated drivers. Drivers are the bits of code that tell your computer how to use its hardware - like the part of the brain that helps interpret what your eyes see. Occasionally manufacturers of various hardware devices will release new drivers that help their products work better, and may fix known issues with certain games. Make sure to update your drivers regularly, even if you're not experiencing issues.
CLEANING YOUR COMPUTER
Computers get exceedingly dusty inside that case, and the more dust and gunk there is, the hotter the components get, and the harder it is for them to work right. If you have a laptop, you need to get a professional to clean it out for you, but if it's a desktop you can do it yourself.
Here's how to give your computer a basic spring clean:
To start off, you need a hand-held vacuum cleaner, or an extension for an upright vacuum which gives you a small nozzle or slit to use - think of the kind of thing you'd use to vacuum out your car or the top of a cabinet.
Making sure that you unplug your computer first! If you have an area of your house without carpet, especially if you have wood floors, work there. Make sure you have plenty of space and nothing lying around that could accidentally be knocked or fall into your computer. Start by vacuuming the fans at the back of your computer, particularly the power supply fan. Keep going until there's little or no fluff coming out of them, it might take a while depending on when they were last cleaned.
Now you need to figure out how to open your case. For most cases, one of the sides can be removed. You can find information on how to get inside most cases if you do a simple google search, or look on the manufacturer's home page. Make sure not to touch any of the components inside, and if you think you're likely to brush against something by accident, make sure you're wearing an anti-static strap connected to the case, or are keeping hold of the case with one hand (or foot, if necessary) - and bear in mind that if you do this, you'll need to keep the computer plugged in, but with the power off.
Once you have the case open, get vacuuming! You need to get the fluff and dust out of the fans, the corners of the case, and off of the motherboard and other bits of circuitry. Don't be shy! You can get up pretty close to the components without having to worry about causing damage. The only things you definitely shouldn't do are drag the nozzle across components, and try to pick the dust out with your fingers. Keep on vacuuming until there's as little dust lying around as possible. You may also choose to use a can of compressed air to loosen dust that the vacuum may otherwise miss. Especially for fans, it is usually more effective to use a can of compressed air than a vacuum alone. If you're in a smallish room you might want to open a window - you'll be surprised at the amount of fluff you'll find.