Is going slow more efficient than going fast?

Speeds 1 and 2 are equally efficient in a given direction. Going backwards will consume the same amount of fuel per tick as going forwards, but you will be significantly slower.

See also this answer -- note the "Lights Off" data showing that half speed and full speed take you equally far on a single unit of fuel.

In my opinion, answer is no. Please read this article, here you have nice example :

I traveled from Fallen London to Cumaen Canal...[]... Here are the resulting data:

• one engine power = -2 fuel, -4 supply, -9 terror, time for voyage is 5 minutes, 24 seconds

• two engine power = -2 fuel, -2 supply, -3 terror, time for voyage is 2 minutes, 39 seconds

• two engine power while pushing full power to engines = -5 fuel, -1 supply, -2 terror, time for voyage is 1 minute, 33 seconds