250 timber in 10 min with level 3 Lumber Mill

This only works with Lumber Mill level 3, because there is much Large Timber which provides ~20 timber each.

Nagrand is a zone with an easy long distance view, and it is possible to spot timber far away. You can use this to plan ahead by looking around to see where you should go next while you are running to a timber. The timbers spawn in certain spots, so maybe an optimal route can be found, but i personally just think that running to the closest timber is the fastest way since they have fast respawn time. 

Area to farm timber can be found here.

Extra: Items like Aviana's Feather can be used to get a good overview where all the timbers are, and goblin glider can be used to make shortcuts over cliffs etc.