Windows 8.1 running games very slow

I bought my iMac new in late 2013 and recently put a copy of windows 8.1 on it via bootcamp. However, when I tried to play any game on it which uses 3d graphics it is unplayable due to lag. When I launched Fable: The Lost Chapters (a game that came out 10 years ago!) I get a pop up saying that my machine is not powerful enough to play it. Has anyone experienced similar issues with gaming on windows 8.1 via bootcamp?