Working but sluggish

[cram01]

Hello everyone,
I have a Dell XPS 13 9360 and I am using ist with the Dockingstation D3100. I have connected 2 NEC Multisync EA221WMe via HDMI-DVI connectors. I experience the same problems: every software based on the chrome-engine (Chromium, Chrome, Vivaldi, Atom) is very slow. But only on the external monitors. If I use the same application on the laptop-screen it works fine. If I put it onto one of the NECs it is becoming unusable. Other applications are free from these problems but I admit, that there are slight delays as well which is noticeable when writing in textareas in Firefox for example.
One thing is really weird: When the window of the Chrome-Browser reaches into the laptop-screen, the slowness disappears on the NEC-screen!

I have installed ubuntu 16.04 and the displaydriver version 1.3.52. The USB-port is installed as USB 3.0 device. Network, keyboard and mouse connected to the same docking device are working fine.