Tonyrh,
Root cause could be the way that Kodi mounts SMB shares. Sometimes the access to these Kodi mounted SMB shares is fast and sometimes it is slow. It varies between which operating system is accessing the SMB shares. I had a similar/same issue and posted about it in Kodi forum. see the thread here: Kodi forum post
In short, the main way to get always fast access to SMB shares in Kodi is to manually mount them as described here: Mount Network Share
If you do this, you should always have fast access, no matter from which operating system and machine. If you still have problems with speed then I would not know what the problem is and how to fix it. Just make sure if you mount the SMB shares manually to remove them from Kodi first (by removing them inside of Kodi).
I think you misunderstood my problem: the hard disk with the media files is physically attached to the Pi and mounted by automount (autofs); the Pi is then sharing those files over the network, using the smb server implementation in Libreelec.