Albany to Bennington VT and over to Brattleboro is a pleasant drive. From Brattleboro head to Keene NH then choose between heading to Nashua via Rt 101 then up to Manchester NH and east on Rt 101 to connect with I95 north to Maine. Between Nashua and Manchester it's quicker to connect with Everett Turnpike to then back to 101 which at that point it a multi lane divided hwy.
Other option from Keene is to head to Concord NH connect with I89 then I93. Go south to 101 for quickest route. Go north to I393 then Rt 4 for scenic route (very pretty section between Durham and Portsmouth) to Rt 16 and then connect with I95 N. This route avoids a couple of toll booths. If you go via Nashua you will go thru Bedford and Hooksett toll booths.

This could add more than a couple of hours of driving. I hate I90 but if you head that way you can exit early around Springfield MA and take I91 north to VT. Get off in Brattleboro and follow above directions. This avoid so much interstate driving and is our choice of route home when we've been in PA or NYC.

BIL used to go from Concord NH to Portland ME a lot and said best route was from I393 to Rt 106 head to Conway NH and then east to Portland ME to connect with I95. Conway to Portland is about an hour. I95 can get pretty boring.

My favorite route is Bennington-Brattleboro-Keene-Concord then taking 202 over to Portland, stopping at Shaw Ridge Farm near Sanford for ice cream. (Sometimes we go this way for no other reason than to stop for ice cream.)

My only concern is that you use the word "efficient" in your post. Anything other than the interstates will not be efficient in this area. It is very common to get behind a slow-moving truck on the state and local roads over the mountains, and passing is not always possible. That being said, in my experience, the non-interstate routes would not add more than one to two hours to the drive which you said is acceptable. Time and day of the week would make a difference also, especially the closer you get to the coast; Friday eve and all day Sunday are the worst. (Then again, traffic can back up on the interstates at those times in the summer too!) Any extra time you take getting across VT and NH can probably be made up once you hit I95 in Maine, on which people regularly drive 80+ mph.

Good information, though you should also look at passing north of the White Mountains (these routes take you south through more builtup areas).

Look at US 302, for example, from I-91.

If you are a person who likes a drink, NH prices are probably the lowest in the country. You will have customs limits going into Canada, but you could take some and buy more if you come back. Ordinary prices in NH are better than duty free shops, and they also have sales.

Than you all so much for replying --- I'm taking a look at your proposed routes, since I really have no clue! Following slow vehicles can be annoying, but if the scenery is better and I'll only lose a couple hours, it's worth it!