Problems with lock team

[luxlunae]

Right now I'm four boxing rather than 5, so that might be part of the problem, but my locks just feel really slow.

I'm running 3x lock, with a shadow/disc priest and they just feel a lot weaker than the other teams I've levelled up (at level 55ish now). Right now I have 2x demonology running with felguard and 1x affliction running felhunter (I thought the mana buff might be nice) but it seems to be working terribly for me. I've tried a variety of things, but double dotting everything doesn't actually kill it very fast and 2.8 seconds is a REALLY LONG nuke. The shadow priest blows them away and even disc (in instances) she's often competitive. Any levelling faster hints for locks? (the last time I played a warlock below level 80 was three years ago). They aren't yet at a level to pick up incinerate so I thought shadowbolt spam was the way to go but its still taking at least two-3 casts to get anything dead. And shadowbolt is long casts.

edit: I generally run with simple two button spam for levelling and I've tried a variety of things for that two step.
edit2: they all have chest, shoulders, helm, back heirlooms and one of the locks and the priest have heirloom weapons