Your scan-time depends on many parameters like Port-List, Timeouts, e.g. if you scan all UDP ports every host might have “65565 * UDP Timeout / Max-Paralell per Hosts” then your host-alive criteria is very important, a best practice would be a ICMP-Ping Alive, the default might detect running ghosts hosts depending on your network.
If you have for example Proxy-Arp, that will ruin your scan time. So you need to know what Ports, UDP will slow your scan massive down, TCP and ICMP Ping Alive might be faster but if you need to discover services behind a firewall you need other settings.
The default values are a best practice from the security point of view not the speed !
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