Re-Launch the scan nmap -T4 --max_rtt_timeout 200 --initial_rtt_timeout 150 --min_hostgroup 512 -P0 -p80 -oG pb2.gnmap 216.163.128.0/20 Starting nmap ( http://www.insecure.org/nmap/ ) [...] Nmap run completed -- 4096 IP addresses (4096 hosts up) scanned in 868.714 seconds Much better than 1236s, but leaves room for improvement.