Slow recursion with ipv6 enabled?

[Job]

Hello,

on Bind 9.10 (latest version of this stable branch), i notice in some cases a relevant slowdown when resolving (for the first time) hostname, when named is launched with both ipv4 and ipv6.
It use recursion to fetch for the first time the information and i have, often, about 2000/3000ms of slowdown.
Then, when record is in cache, it is fast.

By launching Bind with only ipv4 (-4 switch) also recursion is very fast.

Do you think there are some known issues or i have to refine my configuration?

Thank you,
Francesco