I found out that the fans spin up only when the cpu is for a little time at 100 degrees. This behavior is the same under windows, wondering if this is how it is done today, b/c normally those temperatures have been considered bad for hardware lifetime. But it seems to be intentional. So it is boosting continuously with a package temp of ~100°, with at least core 3 continuously 100° and the other around 89-98° (celsius).   The problem is solved by now see [URL] the throttling paragraph. I can't tell you the exact reason yet though why this works.  
Hey there, since I got this machine, I had to solve a lot of problem, including reporting new kernel bugs. This one thing is left now, which is that the fans bareley spin up when in performance mode. In windows it is fine, but under linux they barely spin it all. This comes up in performance as well under windows there seems to be a permanent boost active which it can pretty much maintain, under linux that boost is canceled just after a a few seconds (usually 2-5). I think those two things are somehow connected, since the linux machine should be able to boost the same way windows can, but of course if the fans never come up it wont. The dell-smm-hwmon module is faulty and the fans are not detected by linux at all, that is the first problem, but the bios should still  take proper control of the fans normally, which it obviously does not. No matter if I blacklist the hwmon module or not.   So coming here, hoping someone either from the Dell team or someone from the community might now an answer. This could be a hardware/firmware problem, but also kernel problem.
XPS 17 9710 - fans don't spin up properly under linux