Slow ethernet speed from router to desktop

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
   - average speed 394 Mbit/sec

stable openwrt-19.07  from
   - could not be tested, package iperf3 not available

OpenWRT 18.06.4 with some tweaking ( 
   - 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?