Finder "Preparing to copy" to SMB share - Big Sur...SLOW

This is driving me mad: When I copy a folder to an SMB share, I get fast, snappy performance. If I try to copy the same contents in that folder, but as select-all files, Finder hangs on "Preparing to copy" for a while and then the actually transfer is 1/5 the speed. What is going on?

Details:

• I have an M1 Macbook Air on 11.5.2 connected to a ZFS NAS over 10-Gigabit. Testing the connection via iperf give me close to full 10-Gigabit bandwidth, all good here. Testing Black Magic Disk Speed to the SMBD share gives me close to 1 Gigabyte per second read and write (almost full 10Gig), all good here.
• The test folder contains 427 large images totaling almost 7GB in size.
• Copying the folder, by dragging the whole folder, to the SMB share begins instantly and transfer right around ~550 MB/s. Understandable difference from the Black Magic test because this is many "small" files as opposed to one big one.
• Copying the same files, but by selecting all the individual files, and dragging them to the SMB share incurs a long "Preparing to copy" (30 seconds) and the the transfer only happens at ~95 MB/s. (bad)

Can anyone please explain this and explain why the behavior is so different for "whole folders" versus "lots of files"??? Or at least let me know if they experience a similar thing with a similar test?

Thanks!