A week before we upgraded Magento 2.2.8 to 2.3.3 edition. Post that we are facing huge performance issues. Existing version was run with 4 frontend nodes but post upgrade it takes 10 nodes to run. If we try minimize the nodes then site leads to 504 error. Do we have any specific reason for that? Following screenshots took from NewRelic reports.
I'm not aware of such vast performance losses by simply updating magento itself.
With magento updates sometimes system requirements change as well. So I would start checking changes to the underlying images and settings. Eg. if you upgraded to a higher php version I would make sure that xdebug was not installed in the process. From my experience it degrades magento performance at least to half just when showing up in the loaded module list.
If there are no relevant system changes and magento configuration settings haven't changed during the process (eg. caches, application mode) I'd probably check for timed-out requests, and try to narrow down specific page types or blocks that render much slower than before to isolate the problem.