Not a performance issue per-se, but more an ugly unintended consequence of IPv6 issues: peer-flooding in torrent transfers. I use libtorrent to distribute my data sets and i'm seeing all the usual trackers are being flooded (DoS-like) via the vast available, in this case phony, IPv6 address space[1]. The unstated contention by agents of content owners being that bit-torrent can never do anything but wrong. Selfishly focused on their own content they're statistically not utterly wrong, but they are using the design of IPv6 to poison the well for benevolent use.

[1] https://torrentfreak.com/popular-torrents-being-sabotaged-by...