Complex vs multiple simple mesh performance

Hi guys,

I was just wondering if anyone could help me out a little with meshes and their impact on performance?

The particular object i'm working on atm is a cog that interacts with other cogs using physics. So imagine the complex mesh is a cylinder with 20 sides and 10 of those side have been extruded to make a simple cog. Would that involve more cpu time than say 10 boxes that have been arranged to fill the same area?

I understand that I could just script the rotation but i'm curious about how meshes affect performance.

Thanks for any advice and help!

EDIT: here's a pic to illustrate. The cog has an extrude in the centre in this image.