Variable speed pump motors hold the efficiency fairly well at lower RPM but the drive electronics affects the efficiency at very low RPM which is why 1000 RPM seems to be the sweet spot for efficiency. After than, the drive electronics start to dominate the energy use so efficiency drops. For example, here are a few operating points and the loss of efficiency from full speed:
3450 RPM, 2830 Watts
1725 RPM, 422 Watts, (17% loss of efficiency vs affinity eqs.)
775 RPM, 87 Watts, (63% loss of efficiency vs affinity eqs.)
So at half speed it is better than a two speed efficiency drop but at lower speeds, it gets worse.