I replied to this before?? OK, maybe I goofed.
Yes, it is slower. The lens chatters - that's the best way to describe it. It searches back and forth very quickly, not like a lens hunting from one end of the range to the other. In less than a second it locks. But that can still be a long time! It is still faster than focusing manually of course and tons better than trying the Canon 1.4 which doesn't even try.
I've heard of many who use this combination for the same reason.