When using skills that launch the player (eg. Burning Retreat, Burning Speed, Ride the Lightning), does Swiftness increase the distance traveled by the skill or reduce the time it takes to travel the skill's distance? Or does Swiftness have no affect on the skill?
I haven't done comprehensive testing, but the rule of thumb seems to be if the movement skill temporarily makes you unaffected by gravity1, movement speed boosts will increase the distance a skill moves you.
What seems to happen is that for the gravity-defying portions of movement skills "push" your character at some multiple of their current speed, while non-gravity defying skills apply a fixed speed vector in the direction of action2.
Note that some of the skills have both types of behavior! E.g. Rocket Boot's initial blast animation is of the gravity-defying type and affected by movement speed, while the secondary roll you do is not. Another example is the Ranger greatsword Swoop skill - it's initial movement is a ground rush, and then a leap.
1Go to a cliff and use the movement skill in question. Do you go out in a straight line for a period of time (or even upwards) or do you immediately start falling? For example, RTL you immediately are affected by gravity. With Rocket Boots, you are not affected by gravity until near the end of the skill.
2I guess this is to get more consistent behavior out of the leaping skills? If you were moving down a hill slope, simply applying another movement vector might not be enough to make you "leap." Not sure why they don't just do a fixed movement vector upwards though.
Swiftness has no effect. Taken from the GW2 Wiki: Swiftness is a boon that provides an increase to movement speed. Other effects that provide a bonus to movement speed (e.g. certain traits and signets) do not stack with swiftness; only the highest increase in movement speed will be applied.
This means that swiftness doesn't quicken the cast time or the distance traveled or the increase the distance.
Hope this helps.