K-bobster Let's make sure this isn't a save or settings issue first. Move or rename the Sims 3 folder in Documents > Electronic Arts, so the game creates a new one. Start a new save in Sunset Valley, jump into building, and let me know how it goes.
Build mode always lags a bit eventually, but it shouldn't lag so much or so quickly. If that still happens in the new save, I'd like to see a dxdiag from your computer. Click Windows key-R, enter dxdiag in the run box, wait for the scan to finish, click "Save all information," and save the file to your desktop. From there, you can attach it to a reply using the paper clip (Attachment) icon included with the other formatting buttons.
Please also let me know whether you've taken steps to manually limit your in-game framerates.