As stated earlier in this thread (see page 2) IS does not slow the AF system and will likely help it. Look for the Chuck Westfall quote (HT. privatebydesign). This makes sense, particularly for moving objects. The steadier the image, the easier to focus.
Mine is on at all times for all IS lenses in my sig line below. It is curious though that experiences with IS, particularly with battery drain, must be dependant upon the lens. I think my biggest power hungry lens would be my 70-200 f2.8L Mk2 and I don't think there's a noticeable drain using IS on this lens. But, maybe there is with the really big boys.
[Edit: Just realized that I only have two IS lenses in my sig line. Others include the 18-135 (non-STM), 17-85, 70-300 (non-L).]