What does your sidekiq queue look like? It could just be rebaking all posts and load calm down eventually once the backlog has been got through. Ie the situation should be transitory.
This has happened to me a few times recently. Every time I’ve manually rebooted then performed a command line rebuild. Obviously not a good solution long term and risky.
This is a sign your machine no longer has enough memory. Consider increasing swap to cope with the spikes.
I’m finding I need a 4GB machine plus 3GB swap now to execute online updates: how much memory do you have?