How to solve high CPU usage after reboot

Firstly, I don't know, I'm not sure what updatecheckerd is.

If everything is working ok besides the high cpu after a reboot, and I assume the cpu goes back to normal after a period. I would guess it's some sort of syncing going for HA, and the syncing might be unavoidable.

I would look into tuning postgres, though. It's possible postres isn't making effective use of available memory so has to do things in a cpu intensive way to compensate.

check out https://pgtune.leopard.in.ua/#/ and docs on tuning postgres

I had the same high cpu use on my server, fixed it whit

vm.overcommit_memory = 2 in /etc/sysctl.conf