I have always been playing the Acrobatics skill for jumping so that every bonus to speed automatically affects the Acrobatics bonus to jump with +4 for every additional 10 ft. of speed. However, now I have been made aware that the rules about the Acrobatics actually say (emphasis mine):
Creatures with a base land speed above 30 feet receive a +4 racial bonus on Acrobatics checks made to jump for every 10 feet of their speed above 30 feet. Creatures with a base land speed below 30 feet receive a –4 racial bonus on Acrobatics checks made to jump for every 10 feet of their speed below 30 feet.
Not that it says base land speed. That would mean, for example that the fast movement class abilities of the monk or the barbarian do not contribute a bonus to their Acrobatics checks made to jump (as they do not affect the base land speed), and neither would temporary effects like haste. On the other hand, expedition retreat and longstrider explicitly affect the base land speed and thus would grant the jumping bonus. Also, encumbrance reduces speed, but not base speed and thus, a character slowed down by encumbrance would not get a penalty to jump.
I wonder if this interpretation is correct?
A similar thread was opened some time ago HERE.
James Jacobs commented on that:
I'm 99% sure "base land speed" means the same as "land speed." But it applies to your land speed at the time you make the jump. You wouldn't get the bonus when you're standing still.
I agree that it's worded awkwardly, but in cases like this it's best to apply a dose of logic and common sense; if someone were moving faster than someone else when they jumped, logically they would go further than the slower person.
You can find his original answer here.
I think this is the closest thing to an official answer to your question.