Would I be right in assuming that you have distributed pollers for that amount of hosts, or are you running a single server?
If you have distrigubted pollers then you need to add the -g option to the ping, i.e.
* * * * * librenms /opt/librenms/ping.php -g POLLERGROUPNO >> /dev/null 2>&1
If you only have one poller than it may be down to high load on the lnms server.