I also saw a large drop in performance across my LAN on my MT7621 ER-X gateway - down to a bit under 300 Mbps going from 19.07 to 21.02 with the ER-X as the server:
https://forum.openwrt.org/t/openwrt-21-02-0-third-release-candidate/99363/125?u=eginnc
The MT7621 target was very recently switched to the 5.10 kernel in snapshot, so I just gave that a try and am happy to report things are looking up for the old ER-X: ~500 Mbps without any flow offloading enabled and packet steering checked. The four CPU threads peak ~74%, ~90%, ~35% and ~70%; so the load is getting spread around fairly well.
I saw no further improvement from enabling software offloading. Still, I'm pretty excited to see improvement to half a gig without offloading on this hardware!
It looks like offloading may be back in the next release:
https://forum.openwrt.org/t/openwrt-21-02-1-first-service-release/110461/119?u=eginnc