Canon 17-85mm IS "glides" into focus?

I just received a Canon 17-85mm IS today. I have a Sigma 18-125mm, and recently sold a Canon 28-135mm IS, having used that lens for about a year and a half.

In trying the new Canon today, it seems that the lens slowly "glides" into focus, for lack of a better description. In trying it side-by-side with the Sigma, the Sigma (which is no speed demon) snaps into focus in situations where the Canon responds noticeably more slowly. Although I sold the Canon 28-135mm well over a month ago, I'm pretty certain that it responded much more quickly than the new lens is. The new lens is functioning at the same slow focus speed regardless of whether or not I have the IS turned on.

I'm assuming I just ended up getting a defective lens, but wanted to see if anyone else had a similar experience before I go through the hassle of exchanging it.

Thanks.