Down the road, 3x Felguard with 2/2 Improved Health Funnel is very potent. You won't be able to have Felguards until 50th or so. There is a guy who 4-boxes Felguard Warlocks and was able to level from 61 to 67 in instances (Ramparts-Furnace-SlavePens-ManaTombs) without a healer or 5th party member. The amount of healing is insanely good, and they deal decent single and aoe damage.

As mentioned above, SL/SL is an extremely efficient grinding spec. Instant cast Agony/Corruption/Siphon Life, probably with a Succubus with specials off except for Soothing Kiss so she doesn't take threat and has mana for the Dark Pact. Little downtime, if any, especially with three warlocks sharing the mana spent and therefore mana regeneration between them.

Many will argue an affliction build is the least downtime because of the self sufficiency. Between Dark Pact and improved Life Tap your mana regeneration is great. Agony/Corruption are increased in potency, add in Siphon Life, Unstable Affliction, Haunt, Immolation and instant-cast Howl of Terror.

I'd argue Affliction as the fastest solo leveling spec, but would probably go with SL/SL until you can get Felguards as a boxer. Felguard is the least amount of clicking to be effective as a warlock, which makes it the easiest to be used effectively as a warlock. No matter how you spec, with three warlocks sharing the damage dealt and therefore each regenerating (or tapping) mana, downtime will be greatly reduced compared to any single-boxer spec.