Without more information, it would be hard to help.
Some effects (especially temporal ones) can only work with one thread at a time. Some smaller processors (such as the one in your iMac) are intended to work more efficiently in a single-threaded manner whereas some more powerful setups (such as my dual Xeon processors) are optimized for multi-threaded work. The irony is that some things will be faster on systems that have less power. Granted, most things should be faster, but sometimes that one little effect can throw the whole composition into single-processor usage.
To help you further we would need to know things like exact version number of AE and your OS, what format and codec of any footage, and ESPECIALLY what effects you've applied. Time-remapping, Roto Brush, etc.?