From my most recent run without Heirlooms I have so far shown myself that an Undead route may be the fastest ( of course I will build on this no doubt). You have the starter area with a plethora of enemies, and if you play a class such as Hunter or Priest you can move and attack.

As you reach the second area you are met with 9 different rares in the immediate area; this of course I am determining the make or break point for any run. If you get the rares then you get the better time. My most recent run I had a few mess ups, and they can be improved on. I'll continue to test other races to see what else may be either faster or on par with the Undead starting zone.

I haven't checked, but with the new hunter rare pets that show up in Elwynn if they give rare mob levels of EXP as well, it might be a tighter time. I'll likely get back into the races at some point, some other things on my plate as of right now, but good run either way.

I think it'll eventually come down to just depending on RNG and killing a specific number of mobs between rare spawns and using the bike in the downtime because the walking travel time is what really starts to add up, moreso in the last two levels.

EDIT: Also, it was a good move putting the trike under favorite mounts beforehand.