Speed

Speed is a status effect which increases an entity's walking speed by 20% multiplied by the effect level. It expands a player's field of view (FOV) accordingly. Negative levels will decrease speed.

Unaffected mobs
Boss mobs are immune to the Speed effect. While not immune, flying mobs are unaffected by the Speed effect. Shulkers are also unaffected by, though not immune to, Speed.

Trivia

 * Speed allows slimes to jump farther. High levels of speed can make slimes bounce out of view with just one jump.