Something eating CPU on Sony XPeria XZ, how to find out what it is?

I have two Sony Xperia XZ devices, bought both very recently in UK as used-with-warranty. One is working great, but the other (which has a 3 logo come up when booting) apparently has something using CPU all the time. Why I think so: - Phone is warm even when not used - CPU-Z and the like show frequency not falling under 1200 MHz, and usually over 1400 - while the other has it hovering under 1000 and sometimes hitting 330. - Antutu and especially GeekBench 4 results are worse than the "good" phone So how can I catch the "perpetrator" and get rid of it? AVG and MalwareBytes scans find nothing. And the phone behaved like it was just factory-reset when I started it, so I'm not sure a factory reset would be of much help.