My Mac is currently slow as molasses, when I try to edit the DNG files in my library, because Photos is reprocessing them with the updated RAW support.
( How to Weed out Legacy Media in Photos for Mac - Apple Community)
For example, you should remove all PDF files from the library. Some codecs have been deprecated by the Catalina upgrade and Photos can no longer process some video formats or image formats that used to be supported by the earlier system versions. You may still have legacy media on your Photos Library, that Photos can no longer handle after the upgrade.This can take a few weeks until Photos is back to normal for a library as large as yours. If you just recently upgraded to Catalina Photos may still be upgrading the library.There are several possible reasons why Photos may be slow, when you open your Photos Library.
