If you have a large catalog and use Filter folders, then next time you use Import (or Synchronize or Drag&Drop, ...) it will take a lot more time than usual or essentially freeze Lightroom. Catalogs with 10.000s of photos -> LR freezing for seconds or tens of seconds. Catalogs with 100.000s of photos -> LR freezing for minutes or hours.
Windows 10, Intel i7 6700K. No plugins were active, not even the standard ones like Flickr or Adobe Stock. I have made a lot of tests and this is what happens:
Step by step reproduction
1) Open LR 2) Just for comparison: Import 10 or more files files - this takes seconds 3) Remove those files from the catalog 4) Use Filter folders in the left panel: Write "aaa" there, Enter, then clear the text by clicking on the cross. Filter folders should therefore be no longer be active 5) Import those same files again - if you have large catalog, this now takes much more with LR possibly freezing after about 6-8 photos imported.
How much time it takes depends on the size of the catalog:
With an empty catalog, there is no slowdown.
With a small test catalog containing around 20 000 photos it took around 10 seconds instead of the original 2 seconds without using the Filter folder (LR became unresponsive during the process, that didn't happen without Filter folder)
With my main catalog containing 270 000 photos, the import of 10 new photos causes LR to completely freeze for 63 minutes. (But of course, normally I just kill it after a minute and avoid Filter folders like plague.)
Same freeze happens when using the filtering by color labels.
Greetings, Updates for the Adobe Photography Products were released on 10.20.2020 that include fixes for these issues. Please install the most recent update and confirm that your issue is now fixed. Please let us know if you encounter any issues. Thank you for your patience.