Movement speed how does it stack?
The universal max cap on movement is 33% (Swiftness and Super Speed), and by nature are NOT cumulative with different sources. Different “types” of speed buffs do not stack, though swiftness can stack duration (up to 9 different boon sources) with a max duration of 2 minutes. So in effect, the strongest speed boost currently on the character takes effect, and drops to the next strongest after the first one expires.
Swiftness is also a little odd in that duplicate sources (the same player and same skill) will re-up an existing stack rather then add a new one. This is rare in practice due to cool downs, but can still happen.
On a side note, most movement speed buffs are affected by the In-Combat/Out-of-Combat movement rates. However, 2 buffs, Super Speed and Speed of Battle, will use the OoC movement rate regardless of you being in combat or not. This makes these 2 buffs very powerful if you are being tagged, or are caught in damaging environmental effects.