My first tankadin levelled 10-61 as Ret... this was fast. I also had a friend who was a Shadow Priest at the time who levelled with me probably 65% of the time. We just dps'ed the @#$% out of stuff. At 61 I went Prot and at 70 he went holy, and now we have no problem finding groups for instances/raids.
My alt Pally on another server is 22 now, and has done 10-22 as Prot. Seems about as fast as Ret even tho I primarily solo with him, but he does get a lot more rested exp since I don't play him that often.
Levelling as Prot solo would almost definietly be slower than Ret if it weren't for my rested exp. If you are in a group of 2 or more fairly frequently while levelling, it probably won't make much difference whether you level as Prot or Ret since having groups will just make the quests go that much faster (and would let you practice your Prot skills as you level).
You can always try levelling as a Prot, and if you find it slow, respec to Ret. That first respec only costs 1g....