Slow download speeds in IPv6 tunnel

[atsuko]

I'm running my home net on a Raspberry Pi 4 8GB running Debian, with an LTE
modem as my WAN interface. And because my carrier sucks, I use a VPN to get a
public IPv4 address. I typically get 15-40 down and 5-15 up depending on
congestion with no signal issues. VPN and tunnel servers are both in Dallas,
and I live near both. The VPN adds 1ms to my latency and also guarantees me net
neutrality which is nice. My IPv4-only configuration will run for days at a
time and still work at the usual speeds so I know that's fine.

The tunnel works using the provided instructions, and radvd hands out addresses
from my /64 prefix that also work. I got up to the enthusiast certification
rank too by running a web server on that Pi and adding an AAAA record to my
domain. But my download speeds through the tunnel are awful. Usually less than
1mbps. Upload speeds are typical though. Is this HE's problem or mine?