Today I stumbled upon this reddit-article.
It speaks about using the modesetting driver shipped with xorg instead of the xf86-video-intel driver. I did a benchmark with the xf86-video-intel and the modesetting driver to see which performs better in order to decide if I will drop xf86-video-intel. Here are the results:
Unigine-valley Basic preset @xf86-video-intel:
Unigine-valley Basic preset @modesetting:
It doesn't really matter what hardware I'm using, all that matters is that the situation improved. But for reference: Intel HD Graphics 5500.
So my choice is simple, I'm ditching xf86-video-intel in favor of using modesetting, which was already installed on my system together with xorg-server.
because of the switch to modesetting driver the xbacklight utility does no longer work. Using a xorg config file with the Backlight option set to intel_backlight also does not work as a workaround. So I installed light-git (from the AUR) which allows me to control my laptop backlight once again.