RTH is a slow driver but you can kick the right stick forward to make it faster or cancel and resume control yourself to make it go faster on the way home.
Yes...
It's set at a speed that is optimum for mAH drained per kilometer flown...
So not battery life per say... but stretching distance to empty
And as Meta4 points out... it can be adjusted on the fly...
Some control apps such as 'FPV Camera' can display amp draw in real time...
In playing with that, I found that amp draw really goes up as speed climbs...
I personally use about 45kph/28mph as a good compromise when flying for range.