From the currently prevailing perspective, it is a "given" that automation reduces pilot workload and improves efficiency.
This is not always the case and depends upon the pilot and the automation design. All automation unless completely autonomous creates workload (called by some '
parasitic workload') it is the work required to understand, initiate and use/monitor the automation. Indeed failure to understand '
what it's doing now' or monitor automation has been the cause of several accidents.
With a pilot who is relaxed flying an aircraft manually it may actually be the case that the parasitic workload of managing the automation is greater than the workload using the learned skill of flying an aircraft manually. At the other end of the scale are the pilots who rely on automation and given a sudden reversion to manual flying (direct law) will be unable to cope with the perceived workload due to lack of (continuation) training - this would seem to apply to probably a majority of younger pilots.
Unless as suggested in SLAST's 'Best Practice' pilots ensure that they have practice in manual flying, the next step will be full automation and reduced crewing as the unpracticed flight crew 'cannot take over' in any case.