Each summon spell originally has a casting time of 10 seconds, so this reduces it to 4.5 seconds.
This casts a buff on the warlock, which can't be dispelled.
Talent improvement
This talent is particularly useful when combined with Master Summoner which reduces all of your summons' cast time by 2/4 seconds and 20%/40% mana cost. This gives a maximum benefit of a 0.5-second cast.
Tips and tactics
One of the best uses of this is when your Voidwalker is near death; use its Sacrifice ability to give yourself up to 30 seconds of protection. Then cast Fel Domination and summon a Succubus, Felhunter or Imp (so long as you have at least 1 point in Improved Firebolt) to help finish off the (hopefully weakened) enemy.
Enemies of the warlocks would have lay people think that the demons a warlock summon are invincible and cannot be defeated. This is, in reality, nothing more than anti-warlock propaganda. As all warlocks know, summoned demons (other than the Infernal and Doomguard...well...maybe not the Doomguard) are more cuddly and fluffy than threatening and a Warlock can often find him/herself in the open without a demon to protect himself.
Some warlocks call this the 'Oh Crap' maneuver...
Patch changes
Patch 3.2.0 (04-Aug-2009): Cooldown reduced to 3 minutes, down from 15 minutes.