I updated the settings so the laptop does "nothing" when I close the lid, and it seems to fix the issue. I have been using MBPs last 12 years and never had to do anything to set it up.
Hello,  I have had issues with my 9720 for some time. Whenever I connect my external display (LG 34" WQHD), the laptop becomes very slow. I mean unusably slow. Tend to avoid disconnecting my external display and use the laptop as a desktop. I tested both, USB-C and HDMI, and it does the same. Everything takes a long time to open, and some of the programs I use daily (Autodesk inventor, Recap) aren't usable. The task manager shows almost no CPU or GPUs usage. To remedy this issue, I usually restart the laptop with the connected external monitor several times, and sometimes it takes a long time to restart. Eventually, it somehow fixes itself and it runs fast. I use my laptop mostly at home so it's not bothering me too much but every time I take it with me a have to go through the whole process of restarting again. Has anybody experienced similar behavior?   I appreciate any help.    
XPS 17 9720, external monitor issues