I've never heard of any reports of slowness from Windows Server 2019. If you turn on spool SQL, do you get any clues there? Or do you think it is something else? You might want to try resetting user files to a clean install. Steps to do that are here (no need to reinstall Oracle client for your case)
If this is a VM, make sure it has enough CPU and memory. I don't know what "enough" means exactly, but you can try increasing it.