Quote from latest Sainz's column:
"My feeling in the car got better through qualifying and a very strong lap in the last run meant I finished P7 – best of the rest. It wasn’t all smooth, though, as we had a problem with the batteries. The older they are, the less energy they are able to recover from the MGU-K, so the data showed it had cost me a bit of time, around a couple of tenths."
So it turns out he actually lost a couple of tenths due to the battery not recovering enough energy, though, as he said, it was more related to the age of the battery itself rather than a supposed inferiority to the rest of the power unit manufacturers. Keep in mind though, the latter may also be true.