SMB Transfer speeds

Have you tried setting

Settings->Services->SMB Client->Mitigate MTU issue with smb2

(you need to set the setting level to advanced or higher)

This would explain the symptoms you describe with smb2. Don’t know whether your mac solution provides smb3.

For performance reasons I suggest you mount the smb shares in the root file system, have a look here Mounting network shares with autofs (alternative to fstab)