When installing Debian 10 (Buster) the desktop/GUI applications are slow. Opening applications (e.g. Firefox, Terminal,...) takes quite long and the system is not usable at all.
apt update && apt upgrade
did not help at all. Apparently it is a problem with either the CPU or the GPU driver. I installed Debian on an Intel i5 + Nvidia GTX build (not sure if the GPU is responsible here as well).
The only info I could find was this, but it did not solve the problem
As stated in the above link, it is a driver issue. Yet, the steps above did not fully solve it. As shown here, 'xserver-xorg-video-intel' should be removed and 'firmware-misc-nonfree' should be installed. After rebooting, the problem was solved and the system is usable again