To your question, affliction is by far the fastest leveling spec, not just as far as warlocks are concerned but faster than any other class. As this contradicts what a few others above me have said let me explain. If you are dotting up a mob while your pet tanks then spamming filler until it dies you will be very slow. This is also the wrong way to play affliction. The correct way is to dot up everything around you as you run through and area as mobs chase you and die. This makes your Time To Kill(TTK) spent per mob only the time you use to apply the initial dots, which will generally be 3-4.5 seconds without haste. That is incredibly low with no CDs required to potentially wait on, and the soul shards you may use are automatically replenished as you dot mobs up.
Demo offers a very unique playstyle and is a lot of fun, but has some ramp-up and doesn’t offer terribly impressive TTK killing mobs one at a time. It can have good AoE burst potential, but I prefer destro(with Bane of Havoc) for that department.
Destro can kill 1-2 targets incredibly quickly, but slows down when you start to include more enemies as you have to replenish embers shard fragments. Bane of Havoc+2Xconflag+2Xshadowburn+RoF is potent for AoE burst(though with some risk) if you feel like it. Unless enemies are already very close together affliction would probably still be faster, but I can imagine niche scenarios where destro can pull ahead.
This is of course specific to questing. PVP and dungeons of course are different animals altogether.
My best advice however is to play what you find most fun. Even the slowest leveling spec will seem faster than affliction as long as you are having fun. I love affliction and find it the most enjoyable in general, but not everyone feels that way so if you enjoy demo/destroy more then by all means use them to level.