AGG might be used for benchmarking your SMP system. Here is how to do it:

1. Download and install AGG 0.2.0+ (remove previous installations, if any).
2. Select some fairly large bunch of images to process.
3. From "New Project..." Select the directory with the images.
4. Copy input directory to output directory. Start your chronometer as you press "OK".
5. The measured time will be with SMP enabled. Now you should modify your agg.cfg file. Under Windows, it is located in the directory, where you installed AGG (e.g. C:\Program Files\agg); and under Linux it is in your home directory (e.g. /home/blargh).
6. Open up the file with your favourite text editor (Notepad is fine).
7. Change the first line ("SMP=1") to "SMP=0" and save the file.
8. Repeat steps 3-4. This will be the time with SMP disabled.

Hope this is fun. Please report anomalies (e.g. SMP taking more time than non-SMP or any other weird behaviour) in the Sourceforge's bug reporting system