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During investigation I found a bug with config.xml being blown up on HA sync: https://github.com/opnsense/core/issues/8911
I'm not sure if that was the reason of high cpu, but I had to reinstall the backup. After that, the cpu and mem load got normal.

Just in case, check if your config.xml is bloated. 

I have the same issue with the same symptoms on my HA backup appliance. The master is ok. Release version is like yours and I updated to it right after having both servers installed. Both appliances run on VMWare.

Did you find anything so far? What version did you use before upgrading?