Well it really depends. I play games on the computer almost every day so I'm quite an expert. But It all depends on what type of game it is. If it is an online game then it shouldn't slow down your computer too much but if its a game you've downloaded then it might slow your computer down quite a bit. But remember, it all depends on the game. It also depends on how much memory and data is left on your computer because different games take up their own space on anyone's computer.
So, Online games:shouldn't slow it down too much Downloaded games:might slow it down quite a bit.