Potion of Vigor

The Potion of Vigor is a potion that gives the player heightened attack speed for 3:00 minutes. It multiplies attack speed by 1.35 at level I.

A Potion of Vigor II will multiply attack speed by 1.7. Getting the Vigor effect at a higher level with commands will increase the multiplication by +0.35 each time. The duration can be extended to 8:00 with Redstone, like normal potions, and it can be turned into a splash potion if you feel like you are attacking too slow in the midst of battle, or if your friends want to get in on the rage action.

When brewed with a Slimeball, it will turn into just the opposite: the Potion of Combat Fatigue. This will divide attack speed by 1.45. The Potion of Combat Fatigue II variant will divide by 1.9, and increasing it to a higher level with commands will add +0.45 to the division each time.

Obviously you won't want to drink the Potion of Combat Fatiuge yourself, but you can use gunpowder to make it a splash potion and annoy your enemies.

Brewing
The Potion of Vigor comes from adding an Eye of Ender to a strength potion [made from blaze powder and an awkward potion (made with nether wart)].

If you then add a Slimeball, it will change to a Potion of Combat Fatigue.