Nmap includes six templates that set different timing and performance arguments to optimize your scans based on network conditions. Even though Nmap automatically adjusts some of these values, it is recommended that you set the correct timing template to tell Nmap about the speed of your network connection and the target's response time.
The following recipe will teach you about Nmap's timing templates and how to choose the most appropriate one.
Open your terminal and type the following command to use the aggressive timing template (-T4). Let's also use debugging (-d) to see what Nmap option -T4 sets:
# nmap -T4 -d 192.168.4.20
--------------- Timing report ---------------
hostgroups: min 1, max 100000
rtt-timeouts: init 500, min 100, max 1250
max-scan-delay: TCP 10, UDP 1000, SCTP 10
parallelism: min 0, max 0
max-retries: 6, host-timeout: 0
min-rate: 0, max-rate: 0
-----------------------------------------...