User blog comment:Nyancat1997/Damn you computer!!/@comment-26744861-20120513101458

Chances are your PC is outdated. Was you able to run the game flawlessly when you purchased it?

If so then I'm guessing your problem is related to one of EA's "patches" or you've built up the game so much that you're running low on RAM - 1.5GB is a minimum requirement; it can easily grow larger. Same thing happened to me with TS2 - one of my neighbourhoods (with all sub-neighbourhoods assigned) slowed the game down a lot.