Steps to reproduce:

1. Install Zabbix Proxy 5.0.14 on OEL 7.7 (kernel 4.14)
2. Install Zabbix Server 5.0.14 (in test CentOS 8 was used)
3. Register Zabbix Proxy on the Zabbix Server
4. Install 2x local DNS servers (in test CentOS 8 + Dnsmasq were used)
5. Register Zabbix Proxy's DNS name on both DNS servers for resolving into local private IP
6. Set the 2 DNS servers in the Zabbix Proxy's "reolv.conf" so that Zabbix Proxy would resolve DNS names via those 2 local DNS servers (including Proxy's own DNS name)
7. In the Zabbix frontend create 7-10 hosts for Zabbix proxy with Agent interface configured with Zabbix Proxy's DNS name registered on the 2 local DNS Servers.
8. Set those 7-10 Proxy host instances to be monitored via Zabbix Proxy itself
9. Stop the "dnsmasq" service on the first local DNS server
10. Observe the queue growing on the server

So in case the first DNS server becomes unavailable host checks by DNS name become slow and queue starts to grow.