The only reason I can see for any inefficiency is that the electrons have to slow down & reverse direction & the logistics required to do that are inefficient plus the beneficial effect of the magnetic flux provided by the electron flow through the superconductor material, gets reversed every cycle.

The 1st problem is losses due to latency in the collapse of the magnetic field then there is the effect of the revers polarity...

I mean can you imagine what the ride would be like in Shanghai's maglev supertrain if the field holding it up reversed 50 or so times per second.

That would be one brain scrambling ride <grin>

Cheers Doug