Thanks for all suggestions. Looking for the best solution, I ran more tests.
First against different versions (clean installs) of LEDE/OpenWRT:
LEDE Reboot 17.01.4 average speed 923 Mbit/sec
LEDE Reboot 17.01.6 average speed 840 Mbit/sec
OpenWRT 18.06.0 average speed 422 Mbit/sec
OpenWRT 18.06.4 average speed 238 Mbit/sec
Those are all download speeds from router to desktop. So, newer versions give worse results. I hope version 19 does not give me 10kbit/sec...
And then I have tested tweaked versions:
stable openwrt-18.06 from https://forum.openwrt.org/t/build-for-netgear-r7800/316:
- average speed 394 Mbit/sec
stable openwrt-19.07 from https://forum.openwrt.org/t/build-for-netgear-r7800/316:
- could not be tested, package iperf3 not available
OpenWRT 18.06.4 with some tweaking (https://forum.openwrt.org/t/netgear-r7800-exploration-ipq8065-qca9984/285/1442):
- average speed 489 Mbit/sec
And tried some other suggestions found on the internet:
Software Flow offloading: no difference
Hardware Flow offloading: not available on r7800
Disable Firewall (iptables -F): slightly better performance (5%)
irqbalance: no change (edited after reply below)
What's next? What do I have to do to get 17.01.4 performance on 18.06.4?