KALI – MASSCAN – Faster than NMAP
This is the fastest Internet port scanner. It can scan the entire Internet in under 6 minutes, transmitting 10 million packets per second.
Masscan scans a port at a time, whereas NMAP scans a host at a time. It reports a port at a time, rather than combining all results for the host. Its Asynchronous so is a 1,000 times faster.
Video of Masscan
In conclusion, to scan a couple of hosts use NMAP.
To scan thousands of ports use Masscan.
The two tools are complementary.
An hysterical introduction to Masscan, detailing the level of complaints they receive for security research, and their response to complainers.