Nmap comes with six timing modes that you can set with options (-T):

• paranoid (0): In this timing mode, a packet is sent every five minutes. The packets are sent serially. This mode is useful for avoiding IDS detection.
• sneaky (1): This mode sends a packet every 15 seconds, and there are no packets sent in parallel.
• polite (2): This mode sends a packet every 0.4 seconds, and there is no parallel transmission.
• normal (3): This mode sends multiple packets to multiple targets simultaneously. This is the default timing mode used by Nmap. It balances between time and network load.
• aggressive (4): Nmap will scan a given host for only five minutes before moving on to the next target. Nmap will not wait more than 1.25 seconds for a response.
• insane (5): In this mode, Nmap will scan a given host for only 75 seconds before moving on to the the next target. Nmap will not wait for more than 0.3 seconds for a response.

In our experience, the default timing mode usually works well unless you want to have a stealthier or faster scan.