You are indeed a Swift Learner with this Perk, as each level will give you an additional +5% bonus whenever you earn experience points. Best purchased at an early level.”— Fallout description
Fallout, Fallout 2, Fallout Tactics, J.E. Sawyer's RPG, and Lionheart
Each level adds +5% to experience points gain. Although theoretically useful, the entire purpose of gaining experience is to gain perks that provide useful modifiers and abilities, rather than accelerating level gain.
The perk has no effect in Fallout Tactics.
In J.E. Sawyer's Fallout Role-Playing Game the player cannot choose this perk until level six. If you do choose it at least once then you can take the other two ranks at anytime after that.
Fallout 3 and Fallout: New Vegas
Grants a +10% bonus to all experience gained. With a level cap of 20, Swift Learner provides 2,090 experience points at level 2, while a level cap of 30, it is worth 6,570 experience points at level 2.
However, the perk is still useless for the reasons stated above: Experience serves to unlock perks. Choosing a perk that increases experience gain is essentially wasting a perk slot, especially due to the fact that there's more experience available in the game without any modifiers applied to reach the level cap and the existence of experience bonuses from difficulty level and Well Rested.
A bug in the game allows the player to select the perk for a fourth time, but it has no effect.