vdr - nice -20
vdr - rtprio 99
should be defined in /etc/security/limits.conf to allow pulseaudio to set it's process priorities. It is run as vdr user.
But sound is still lagging a lot for some reason, more and more with time when watching a channel. Switching channel syncs the sound back to normal. Normal here is -390ms audio delay in settings. Audio is almost half a second behind picture which very weird. This is with vdr-plugin-softhddevice-vdpau-hevc