copying over the lan is very slow. Is it a bug?

I have two laptops running the standard Ubuntu 18.04 LTS (gnome). The one was upgraded from 16.04 LTS, the other is a fresh installation. They are connected to my network via cifs.
When copying a specific directory from my server to the laptops using the command line, it takes a few seconds to complete. But using the GUI, it takes in both cases almost 30 minutes!
Doing the test with another laptop running 16.04 LTS (Unity) using the GUI showed no differences to using the command line. I did not find any possibility to speed up the process in 18.04 LTS.