That's kind of odd! Is the actual game running fast as well? For example, do the animals and guests run abnormally fast? Look at a turtle. They are supposed to crawl really slowly.
If the answer is no, what I would suggest is when you're doing some heavy duty designing, either pause the game or design the exhibit/zoo before getting the animals. Most likely the game is running at normal speed but you just like to take your time designing. (I do, too!)
If the answer is yes, I'm not sure WHY that would be the case. However I would suggest applying the available patches if you haven't already. If you are running a version pre-1.1, it looks like the patch for 1.1 updates the game to run in Universal Binary. That could be your problem, if the software is running through Rosetta (although normally that would cause programs to be SLOWER, not faster!
Here is a good place to download the patches:
Versiontracker