How can I simulate having a slow CPU?
One solution is to use virtualization to have another instance of Mac OS X running on your computer. Many virtualization applications (such as Parallels Desktop) let you decide how much resources an instance can have (e.g. memory or cpu). Lion increased its support of virtualizing Mac OS X.

If you do decide to go with Parallels, there's a useful how to article describing how to set up an instance of another Mac OS X Lion.