To have Quickness is to be fast as fuck, boy twice as fast as your peers, in more ways than one:

• Your Combat Speed is twice as fast.
• Your Attack Rate is twice as fast.
• You Age twice as fast.

These perks come alongside a +2 boost to Attack (ATT) and Defense (DEF), the former of which is quite rare.

Quickness is a Water Buff:

• The Bless gives it to you, though that requires 10 and 1 on your Pretender.
• Three alarmingly-cheap Alteration spells give it to you for combat; Quicken Self at Research Level 2, Quickness (AOE 1) at Level 4, and Quickening (AOE 9+) at Level 8. The first one requires 1, the second requires 2, and the third one requires 2 and 1.
• Several Items, notably all of the (unmodded) Arena prizes, provide Quickness.

Temporary Quickness is generally preferred over permanent Quickness, due to the doubled rate of Aging. Quickness also doesn't boost the rate of Spellcasting.

This is NOT Swiftness, which is similar but only affects running speed. On the other hand, these stack.