I noticed today after upgrading that my SMB performance had decreased by a lot, but not in the way you would expect. The write/read speeds are fine.
The problem seems to only occur whenever you open a media file in a share where there is 1000+ media files stored in the same root folder.
If you take 50 of the media files, create a folder and move them into a folder so there's only 50 files in the folder, then playback will be normal, new smb is doing something which adds a ton of delay.
Doing the exact same thing on 6.7.2 is much more rapid.
You can re-create the issue by having 1000+ media files in a folder and browse next between them. The more files you have in the folder the slower the performance.