Hi Folks...6 weeks ago I sent my Sigma 24-70 F/2.8 to sigma for repair because it was suffering from BF / FF issues....I also complained that that it was a slow focusing lens...This lens is known for being slow focusing because it has allot of heavy glass..It uses an 82mm filter...Anyway, what happened to this lens was not a fault of Sigma, but my fault. I dropped it about 5 feet or so and it was never the same after that...So anyway, I sent it in for repair and spoke with a tech. He told me they have a completely new focusing element for that lens and was waiting for it from Japan...Anyway I got it back today and it is a rocket...It went from being super slow to one of the fastest lenses I have...It is sharp as a tack.....I also found out that my Sigma 135-400 was also fitted with a new focusing element as well and that explains why it is so quick and sharp....Anyway, here are some snap shots I took on my way home...I will take some proper images during the week..... I bring this up because others whom I have conversed with regarding this lens all had the same complain...''It is slow''..
I would suggest contacting Sigma and see if your lens can get this upgrade.
Mine was still under warranty and cost me no money, but a total of 6 weeks with out it including shipping time....