BIND 9.10 fast only on alias IP

[Ict Security]

Dear guys,

i am experiencing a very strange beahviour of Bind under busy peak time.

With a quite important number of incoming DNS queries, response are
really, really slow;
sometimes they even stuck.

If i try to query, in those busy moments, an alias secondary IP
address of the same machine, the response is really immediate!

I have disabled connection tracking and raised up nf_conntrack_max.
In system logs, i do not see any limitations or buffer full.

Do i need to balance incoming connection on more alias IP?
Or shall i change some other parameters which i am not aware at the moment?

Thank you, cheers!
Francesco