Extra Fast is a monster bonus available to certain, powerful monsters throughout the Diablo series. This gives them a hasted effect, increasing movement speed and attack speed fairly noticeably.
In Diablo II, Extra fast unique monsters have a very high movement and attack speed; it is difficult to escape from them. The slowing effect of most magic and abilities will have less effect on them. Extra fast uniques are most deadly if the player is already injured and out of healing items or abilities. They are least deadly to a well-prepared party or player, since they only hit faster and not harder than usual.
One peculiar annoyance occurs when an extra fast creature is of a type which can resurrect its minions; it then does so at a frightening rate. Thankfully, creatures that rely on resurrecting their minions tend to be fairly weak in direct combat and is thus less of a direct threat.
Extra Fast returns in Diablo III, but it is now referred to as "Fast". It works mostly the same way as it did in Diablo II, with the only noticeable change being that Champions can now spawn with this, instead of just Uniques.