I just did some quests in Plaguelands, L55-56 for me consisted of: quest line for the field cauldrons (50K), Andorhal quest to kill Araj (28K), and turned in 30 each of Dark Iron Scraps, Core of Elements, etc at EPL (47K), and 1-2 more quest I'm forgetting. This took about 1 hour for the level, but is a bit skewed since those turn-ins took 0 time and were a huge chunk of XP. Still, maybe 1.5 hours per level from quests, I think would be good.
Next up, the kill quests from Tirion Fordring (25K) and the Scholomance key line -- that one gives almost 40K, involves a trip to Tanaris and Un'goro though, but I'll hit some easy quests there as well as in Silithus. Then Outlands for gearing up and then do some rep grind to honored in instances.

Questing is faster, but depending on your setup probably not THAT much faster. I grinded XP in Stratholme from 48-53/54 to get revered with Argent Dawn (so I can 5-man Naxx trash I guess, lulz?) and averaged about 60K per hour in XP including loot time and the occasional trip to LHC to vendor/repair/mail off junk. Skipping loot - if I could have afforded it - would have doubled the XP/hour. My next group, I will definitely skip looting all trash mobs with.