There are 'event hotspots' all over northrend that can provide an unlimited supply of mobs. Ive tried moving south from the spot listed above to higher level crabs, but I did not find another spot that was instant spawn.

There is that quest in Dustwallow Marsh that Xinn found where you can kill an unlimited amount of mobs as fast as you can talk to the quest giver. http://www.wowwiki.com/Quest:Mana_Surges Its level 35ish and has a lot of prep work. Its going to be really hard to try and beat an instance after level 10. SFK and SM you can get over a level a run and with a bit of practice you're under the 10min mark per level, they do slow down if you stay past when its time to leave. I agree that if you have to run all over the game world for instant spawn hot spots it will be 30-40min getting there for a 30-40min grind for 5-10 levels.