As to replicate pretty much as the poster in this thread - Magento 2.2 often extremely slow, 100% processor usage after setup:upgrade
I have now had to restore a Magento from overnight backup 3 times now. I am working to move to Magento 2.3 from Magento 1.9 but I get as far as installing few extensions and then suddenly Magento will start to slow and spawn loads of PHP processes and send the server load sky high. I went as far as removing every single extension I added that day and still the same. Restore database backup too, same issue. I cannot find a way without having to restore from a backup or a fresh install every time. I don't ever see how I will be able to run this in production environment if it keeps doing this...
I don't understand why it randomly does this.... I'm not an expert. I have ran every possible command and searched heavily for a fix for this issue to no avail.