Thanks for that link mrlamud. I am using VLANs, and I had tried only software, not hardware offload with the recent 5.10 kernel snapshot.
Unfortunately, hardware offload doesn't help either.
With SQM off, and using my ER-X as the iperf3 server, I get ~500 Mbps wired between the ER-X and a Linksys EA8500 AP. Reversing the direction and using the EA8500 as the server, I get ~700-800 Mbps. No offloading, software offloading, and hardware offloading all have the same CPU utilization on the ER-X. Same results with SQM enabled on the WAN.
At least for my set-up, offloading (software or hardware) does nothing at all with the recent 5.10 kernel snapshot. The recent 5.10 kernel snapshot is at least a lot faster without offloading than 21.02.
It looks like kristrev has narrowed the problem down to DSA here:
https://lists.openwrt.org/pipermail/openwrt-devel/2021-December/037417.html