Route 2 is the way to go but time of day is critical, during most of the day, the road gets filled up with slow moving campers and trucks, if you can drive early morning or in the evening the traffic is better (but the wildlife is more present!). The short cut around Rumford and Dixfield can save about 15 minutes (driving from the west, take the bridge over the river just after downtown Dixfield, take a right at the intersection and reconnect wth RT 2 just after downtown.
The other big time saver is to stay on I95 down to Clinton (one town north of Waterville) and cut over to route 2 rather than taking Rt 2 in Newport.
One thing to keep in mind is that there is major construction on RT 2 west of Bethel to the Maine line. It may make sense to take the North Road in Bethel and run parallel to Rotue 2 to Shelburne but North road is decidedly more rustic (it turns to dirt for a portion).
Keep in mind along the entire route is that there are a lot of local police and sheriffs who patrol the route, speed at your own risk!