ipv6 is enabled on both my LAN and on zerotier. I don’t believe my internet uplink is ipv6-capable though.
after some more research and testing I think this might be a linux-specific issue. I ran across this stackexchange question that also said hamachi has a similar issue on linux, which gave me the idea to test zerotier performance between mac<->windows.
surely enough, my iperf3 tests on LAN vs zerotier were pretty much identical, in fact for some reason zerotier seems to even outperform my LAN which is a bit funny.
I will continue to investigate but progress will likely slow to a crawl the lower level I try to go–if anyone with expertise has any insight I would greatly appreciate it!