Detailed Explanation
The faster you move, the more attack speed you gain. Specifically you gain 10% attack speed when you move 10% faster than your base speed.
If you move at 1.4x base speed, you reach the max cap for 1 item, which is +40% attack speed. This is very easy to reach if you move around somewhat normally even without extra movement speed items (jumping, strafing, bunny hopping)
NOTE: There is currently a bug in the game where whenever you enter a new stage, the game re-calculates your current speed to be your new base speed. This is explained in greater details under "How it stacks". It is a pretty significant bug and can make this item feel pretty useless even though its scaling is crazy.