I know turbofans are less efficient than propellers due to accelerating a smaller mass of air faster. So my question here is not comparing turbofans vs other engines, but compare how inefficient is a turbofan operating at take off and low speeds compared to that same turbofan in its optimal conditions?
In other words, how much is the efficiency loss? Are we talking about 5% or about 50%?
Also, why exactly that loss of efficiency happen (again, compared to the same engine at optimal cruise conditions)? Is it just the speed differential between the exhaust and aircraft airspeed, or is it something to do with the compressor/turbine efficiency?