RE: Dell Inspiron 15 Gaming (7567) + external monitor = (performance) problems
I've got the same problem, and I looked into it to try to track down a solution. Supposedly, it's caused by the laptop automatically going into power saving mode when the laptop lid is closed. You should be able to fix this by disabling power saving mode in the BIOS, but I've been unable to find the setting myself. Meaning that this would require a BIOS update from Dell to fix.