Although the notebook seems to be powerful enough, I am having problems when it is connected to the external monitor (via HDMI, I have tried different monitors and cables):
1) When the notebook lid is closed (or the Windows Display settings is set to "External only"), Windows is very slow, all the animations are lagging. When both notebook screen and external monitor are on (set to "Duplicate"), everything is fine.
2) In games, I have a noticable FPS drop (up to 20 - 30 FPS in Witcher 3) when playing on external screen (even on "Duplicate" and "External Only" modes). I can have full FPS just when playing only on notebook screen.
Thanks for posting.
Apologies that your computer is not working the way you expected. Other users with a similar issue as yours have reported success by updating the BIOS and/or disabling Turbo Boost. You might try those to see if it resolves your issue.
Here is some information you may find helpful: http://dell.to/2y6sGN3