Profiling, as suggested in Blrfl's answer, is the more appropriate way to identify whether or not your Mathematica code is efficient. However if you really want to slow down its execution, you may be able to do this by changing the priority of the Mathematica process. I haven't tried this but you may find helpful information or software among these links:

Is there any way to set the priority of a process in Mac OS X?

How to permanently “renice” a process on Mac OS X (or iOS, etc)?

Appriority (formerly Renicer) by Northern Softworks

Some of those discussions are a bit old though, so you may need to experiment or search further to find what works on your Mac and OS X version. Also you may find the technique works more effectively if you set your Mac some other heavy processing task to run at the same time - converting a big video file, say.

Profiling, as suggested in Blrfl's answer, is the more appropriate way to identify whether or not your Mathematica code is efficient. However if you really want to slow down its execution, you may be able to do this by changing the priority of the Mathematica process. I haven't tried this but you may find helpful information or software among these links:

Is there any way to set the priority of a process in Mac OS X?

How to permanently “renice” a process on Mac OS X (or iOS, etc)?

Appriority (formerly Renicer) by Northern Softworks

Some of those discussions are a bit old though, so you may need to experiment or search further to find what works on your Mac and OS X version. Also you may find the technique works more effectively if you set your Mac some other heavy processing task to run at the same time - converting a big video file, say.