Guild Wars 2

Tested empirically. Was outrunning my mesmer ally until she cast swiftness on us both, then suddenly we were running side-by-side. Unless mesmers have an ability that increases their speed by 25% while they have the swiftness boon, Signet of the Hunt does nothing while you have the Swiftness boon.

movement speed caps at 33%, with the exception of the rune set that makes swiftness 40% (also other skills that increase directly above that, such as slick shoes for engineer),

so with soth and swiftness you will not run at 58% run speed

Laaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaame. No point in using that signet then I suppose, when other classes have practically perma-swiftness.

The way it works is that Swiftness NEVER stacks. Swiftness replaces passive buffs, but does not stack with them. The advantage to passive buffs is that they are less hassle, you don’t have to worry about “rotations” or using your moves with the right timing, you just move.