This is among the lowest-speed haste items in the game, as I learned from wearing one for some time. Providing a percentage is tricky, there are so many misconceptions about how haste is calculated that you never know if someone is actually giving a percentage for the (as far as I can tell, correct, but thoroughly unintuitive) formula: new_delay = old_delay/(1 + haste_percentage) or if they're getting it (as far as I can tell) wrong, and using: new_delay = old_delay(1 - haste_percentage).
Anyway, a popular method of showing an item's haste has, for reference, the swirlspine clocked at 16%, the FBSS at 21%, and, say, a sash of the dragonborn at 24%. Make of that what you will.
What really matters is... This item is slow - no two ways about it. It's better than nothing, and it does have AC, but it's not as fast as an FBSS, and a whole hell of a lot less common.
Khoren Stonefall Level 58 Knight Brell Serilis
P.S. I heard for the first time yesterday a rumour that the SS belts that drop now in Kedge are rogue only. Can anyone confirm that? I just heard it third hand. My old belt remains all-melee.
I heard that of a recent patch, the SS belt now has the same haste percentage as the FBSS. This was done to compete with the FBSS. The FBSS has 21 - 22% haste to go along with 0.1 wt. Where the SS belt, now with the same haste, has a wt. of 1.0. Along with the 4 ac. So that leaves, would you rather have the 4 ac at 0.9 more in wt. or no ac and 0.9 less in wt.? The 4 ac is a nice addition to any high lvl melee class. Where as a monk would probably prefer the FBSS, since they cannot carry as much wt. as, for example, a warrior.