Update: I noticed that the spikes seem to happen while using the touchpad. So I connected a Bluetooth mouse and disabled the touchpad in device manager. Now the highest DPC is ntoskrnl.exe with just under 0,9 ms. APCI.exe is down to 0,4ms. Not great overall (my old Dell with an 6820HQ managed everything below 0,5ms), but already a lot better.
Now I would obviously prefer being able to use the touchpad, are there any alternative drivers that I could try? Dell doesn't supply one, I suppose its just the MS Precision driver. I didn't install it manually, it installed with Windows Update. Which also explains why at the beginning just after installing Windows, the number would be ok.