You can't have the OS make the decision for you, where Windows can. Apparently it has something to do with Nvidia not willing to share some of their source with the linux community but the end result is that you either have an overpowered computer that will run 2.5 hours and get very hot and make alot noise, or an underpowered computer that does have a longer battery life but visibly degraded performance (even on Youtube...)
Most distros wont even let you boot into X with a dual gpu setup... I tried every version I could of Ubuntu, Debian, Arch, Fedora and more, in the end the version that seemed to work the best was Fedora. But I still had to heavily optimize it and find a way that was practical enough for me to use. I still tried to live with it for a couple of months but I got really tired of everything breaking now and then because I updated a single package or some cosmic ray shifted a bit somewhere...
And don't even get me started on how Wayland is supposed to be better at this than X... The Nvidia control panel won't even open under Wayland...
Opened an issue on stackoverflow about this some months ago, got maybe 10 people following the question and not a single answer...
So yeah lesson learned on this one. If I had to do it all over I would have gone for a Thinkpad without a doubt. For now the most usable "linux" I can have on this XPS 15 is Windows with the Linux Subsystem enabled...
EDIT: also make sure you laptop screen has the same scaling as your (potential) external monitor(s) or you are in for a bumpy ride...