Problem was that before only two ethernet-interfaces were active. One for the company-intranet (which was gateway for internet) and the second one for some device connection.
After adding some 10 GigE-interface, somehow all DNS-requests were first routed via that interface. Which lead to a delay of 3-4 seconds for loading pages.
Can be fixed via setting a lower metrik to the interface, which influences the routing-table: “network- and sharing option > the specific adapter > “IPv4” > “Settings” > “Advanced settings” > bottom “metric” from “automatic metric” to some lower value. Usually it should be 25, I set 13.
You can check via CMD: $ route print for the current value. The used Gateway should have the lowest metric.