I’ve solved as suggested in other forum post ( [SOLVED] GT520 + linux54 5.4.85-1 + linux54-nvidia-390xx 390.138-2 = absolutely terrible performance) By adding
Section "Files"
ModulePath "/usr/lib/nvidia/xorg"
ModulePath "/usr/lib/xorg/modules"
EndSection
to /etc/X11/mhwd.d/nvidia.conf
(the problem was loading the “wrong” libglx.so
with llvmpipe as a renderer)