The extra work of low pressure tires, I looked up a couple rides to illustrate the effects of tire pressure on a fatbike:

In the summer with 12 psi, I rode 24 miles with an average speed of 15.7 mph (the only summer ride listed)
Today with 9 psi, rode 12.2 miles with an average speed of 10.7 mph
Yesterday with 6 psi, I rode 25 miles with an average speed of 9.4 mph (it was -10F... so there's that to factor in too)
Coming home from Theo (tight technical single-track trails with 3-4 inches of fresh snow) I left the tires at 4.5 psi and rode 12 miles home with an average speed below 9 mph. (I had a pump, but was "lazy" - not going to happen again!)