Stamina is one of the main statistics-based functions of your character along with Mana and Hit Points. Stamina is determined by Dexterity and Focus, which translates into your total Stamina pool. Swing speed is dependent on Stamina. Stamina is depeleted when either your character takes damage, or is holding beyond his/her weight capacity and attempts to move.
Your Stamina pool can be increased by certain Item Properties, such as Stamina Increase and Stamina Regeneration. An opponent who swings a Mace Fighting weapon can reduce your Staminia more than other weapons. Some creatures can drain Staminia, like the Crystal Lattice Seeker and Ethereal Warrior.
Every 30 stamina points decrease your weapon swing speed time by one tick (which is 0.25 of a second) so with 180 stamina (or over that) your weapon speed time is decreased by 6 ticks (1.5 seconds) before the swing speed increase is calculated in.