Samba-Mounter how it is called is using mount.cifs to mount the samba shares.
In order to know why it is so slow I would try maybe mounting your drive manually from the commandline and see this german speaking tutorial for example: https://wiki.ubuntuusers.de/Samba_Client_cifs/#Temporaeres-Einbinden
Would be nice to see if it prints out any errors or warnings when executing and if you are able to reproduce the issue with the slow opening of files with this method aswell.