I'm boosting four lowbies at the moment.

I quested them up to level 8 and then ran them through RFC to 10 and SFK to 20. At first, it was taking three runs to get two levels. Quest XP is on top of that if you can be bothered to go and pick them up.

About level 16, things were starting to slow down.

At level 20, I shifted to Scarlet Monastery. I'm currently farming Cathedral with them and at level 25, it's taking 3-4 clears per level.

My booster is a poorly equipped Rogue (quest rewards, vendor weapons and non-heroic blues) and Cathedral takes me 27 minutes to clear. If I had a decently geared Paladin or Mage, I'd be doing four-five Cathedrals per hour.