I think 14 exists as a test case for having a larger search space that needs to be efficiently ignored. Its not difficult solution wise, but if it is particularly slow that means you're doing a bad job at pruning potential steps. Making sure you're doing good early pruning of the empty chambers.
Yeah, that was kinda what I figured too, maybe something about the "pinwheel" shape makes the pruning harder (since beyond each bottleneck there's a large amount of useless search space.)
Kinda tempted to go play with it now and see if it becomes more obvious...