I did some testing with sloth yesterday when I wanted to have some fun in Crypt of Decay. I did the Carpin Cycle for some tribute items and pulled trains of about 20 to 30 mobs each time. I box a warrior and ranger, but play a shaman. First of all, I have never parsed anything nor do I really know how to so this info is just what I noticed. BTW my warrior has 2.0 main and swiftcleaver secondary.
I cast sloth on my warrior before he pulls, he bring back a nice fun train that normally I would just AOE with the ranger and shaman, but this time I wanted to see how he attacks. It was really sad seeing how slow he attacked my targeted mob. It was almost to the point that mobs all around him were taking nearly as much dps from dmg shield being on and repositles.
In closing, sloth effects swing rates just like lassitude and reptile skin. It's very noticable with tons of green mobs all over your warrior if you want to try it out for yourself.