Hey!
I faced the same issue i.e., auto restart. Essentially some units have a defective motherboard. The only solution is to replace the motherboard. I am from Bangalore and the service center got it replaced within 2 days. The phone works fine as of now. But agreed, not a pleasant experience visiting the service center for a hardware issue.
Regarding 48 MP, yes there is a bit of a lag time compared to the 12 MP mode but that is to be expected. Compared to other devices, I am not sure.