I'm also experiencing slow SMB browsing, mainly in Kodi when doing a library scan for updates. I noticed this after switching off SMBv1 and using SMBv3. I did a test and enabled an identical NFS share and the media scanning was drastically faster, maybe 20 seconds vs ~1-2 minutes. However I experienced weird audio popping issues on the NFS share and switched back to SMB. Going back to SMBv1 isn't an option since it's a dated and insecure protocol. I am able to watch movies, so transfer speed doesn't seem to be an issue.
Some more info: There are 127 movies in individual folders, 18 tv shows in individual folders, so not a ton.
Why is SMB so slow when NFS browsing / listing is near instant?