Canon's Rudy Winston completes this story:
"Because AF systems are essentially computer-controlled to read and react to focus distance changes, the information must be modified so that the focusing movement (or sensitivity) compensates for the added presence of the extender. In the Canon EOS system, this is done by deliberately reducing drive speed when an extender is detected.
Before you immediately conclude that this is a problem,
understand that this reduction in drive speed now corresponds to the effective speed you would achieve with the same EF lens alone. It compensates, automatically, for the reduced distance lens elements in the lens’s focusing group(s) need to move to refocus on a subject, with either EF Extender in place.
Accordingly, overall AF performance remains essentially unchanged with an EF Extender attached, versus the lens’s AF speed without an extender."