Slow page load after updating primefaces(-extension) from 6.2 to 8

Question

I am currently in the process of updating primefaces from version 6.2 to 8.0.8 and primefaces-extensions from 6.2 to 8.0.5

I am following those migration guides

Now if I open any page, the layout has some problems at page load (only for a short moment, looks like styles that get applied after the page is loaded or m)

Also, the rows of my datatables are rendered one after the other (10 rows now take ~2 seconds to be drawn/loaded/...)
In general, the application is a lot slower than with version 6.2

As if caching is no longer working (?)

What I have tried:

• leaving the version of primefaces-extension 6.2
  now the UI is working, but I get some ClassNotFound erros, so not acceptable for production
• updating to a newer version of primefaces (10)
  problem still exists
• updating primefaces to 7.0 and extensions to 7.0.3
  problem still exists
• looked at what changed from primefaces extension 6.2 to 8.0.8
  overwhelmed
• checked that javax.faces.PROJECT_STAGE is set to Production
• webapp version is 3.0

Is there anything obvious, e.g. a setting that one could miss?