I'm wondering if it's actually other things on my router that are triggering the slowdown when these large sets are present, because it's certainly true that nft commands (listing tables/chains, adding elements to other sets etc.) slow to a crawl when these large sets are present. I have dnsmasq configured to populate several sets automatically, so maybe it's that?
It looks like at least one other person has noticed the poor performance that I described in my other thread, and they also were able to get the netfilter developers to fix at least some of the issues:
I think I'll just have to wait for the next version of OpenWrt before testing again to see if things have improved.
Edit: Just as proof that I'm not imagining things 
, when the performance tanked yesterday there were errors like this in my syslog:
Thu May 2 16:01:03 2024 kern.err kernel: [605087.575869] rcu: INFO: rcu_sched self-detected stall on CPU
Thu May 2 16:01:03 2024 kern.err kernel: [605087.581663] rcu: 0-....: (5999 ticks this GP) idle=235/1/0x4000000000000000 softirq=11246206/11246206 fqs=2999