Why does Zabbix slow down WEB checks on IPv6?

Question

Monitoring

Hello!

I have CentOS 7, on it Zabbix 3 (of turnips), as well as web check for one site. Site loads < 1 second. This is seen by Zabbix, but exactly until IPv6 appears on the server - after that, on monitoring + 8 seconds.
If you do time curl -6 site.com, then the time will match the time curl -4 site.com (both with and without IPv6 on the server) and this time is always less than a second.

Question - why does zabiks see, with active ip6, the site loading time is about 8 seconds, instead of the expected one?