Download performance issues with IPv6

Hi all,

I'm running OpenWRT 21.02 on MobiPromo CM520-79F (Qualcomm IPQ4019). I have no problems with IPv4 but terrible download performance, 1.79Mb/s on IPv6. In contrast, IPv6 upload is 43.4Mb/s and IPv4 is 76.2Mb/s down, 81.6Mb/s upload.

My WAN provider is Consolidated Communications via fiber and using RA and NDP-Proxy in relay mode.