How does the efficiency of a turbofan compare between take off speeds and optimal conditions?
To compare the efficiency we need to use the thrust specific fuel consumption (TSFC). It's the fuel per unit time per unit thrust.

(Georgia Institute of Technology)

The lower the y-axis value (TSFC/TSFC0), the more efficient the engine is. TSFC0 being TSFC at sea-level and standstill. Speed increases TSFC, whereas altitude (more precisely: decreasing ambient temperature) lowers it a bit.

Taking the extremes from the example above:


M0.4 at sea-level is a ratio of 1.45
M0.8 at 11 km (FL360) is a ratio of 1.675


On ground a turbofan engine is more efficient than in cruise when it comes to fuel consumption per thrust given.

But if you're comparing different airplanes, then the distance covered is also taken into account.


  Fuel consumption per [distance] can be a more appropriate comparison for aircraft that travel at very different speeds.


(Source)

For supersonic jet engines it's different as shown above.