Macro's in Excel from Microsoft 365 EXTREMELY SLOW

I do not know the answer but our suspicion must surely be that the two problems are related. Perhaps your "create pdf" button performs an operation that is fast when the screen does not require updating but slow when it does? I really cannot understand how screenupdating=FALSE cannot "work well" - it either works or it doesn't. There is no room for it working badly.

From poor memory, SceenUpdating is not scoped, ie will not reset to TRUE on its own accord. The error in VBA more often than not is to fail to reinstate ScreenUpdating. That said, I cannot recall how Excel deals with a VBA macro exiting while ScreenUpdate remains stuck at FALSE. Surely, Excel cannot tolerate such an exit.

So my suspicion might be that your macro is somehow transitioning between in-macro and out-of-macro state, implicitly setting ScreenUpdating=TRUE whenever transitioning out-of-macro and failing to reinstate ScreenUpdating=FALSE because you do not recognize the transition back to in-macro.

That might explain your language "not well".

Are you using Excel's built-in PDF creator?  I would focus my attention at that PDF creator.  It seems to interact with your Excel in an unhealthy fashion, differently on O365 than your other version.

Once more, I would comment out the one-line call to the delegated PDF creation to localize the problem.  If performance improves, and if the problems with ScreenUpdating somehow go away, you know that both problems relate to that delegation.  (The actual PDF creation is unlikely of any relevance to the rest of your code.)