Just looking at a server where the NodeDiscoveryPoller queue is over 600000. I can see in the logs that it is trying many many IP address starting with 172.
I found that I had added a discovery target of 172.28.15.0 with a netmask of /0. I don't actually know what this means to NetXMS, was it trying from 0.0.0.0 to 255.255.255.255 inclusive?
I think a warning that a netmask of /0 means way more IP addresses than you'd usually want would be a good idea here.