[koomi]

DS-Lite[0] means you have proper native IPv6 but only a tunnel on top of that for v4, so it's unlikely that IPv6 connections are negatively affected by this setup.

[0] https://tools.ietf.org/html/rfc6333

Edit: On the other hand, if connections via IPv4 were spotty I would not hesitate to suggest problems with NAT or path MTU.