On a Pi3 it was possible to improve this by messing around with "vcgencmd arbiter set arm_uc" command, e.g.:
# vcgencmd arbiter status
# vcgencmd arbiter set arm_uc 12 0
No idea it this is still valid for Pi4.
Is there anything else I could try? I also tried to connect dvb card to usb3 with no improvement.
I'm running now le 9.2.1 with latest firmware, eeprom and usb3 firmware.