If you need to speed up your scans a little bit, you can always choose to disable reverse DNS resolution for all your scans. Just add the -n parameter.

[root@mmzacademy:~]nmap -p 80 -n 8.8.8.8Starting Nmap 7.60 ( https://nmap.org ) at 2018-10-01 09:15 -03Nmap scan report for 8.8.8.8Host is up (0.014s latency). PORT   STATE    SERVICE 80/tcp filtered http

See the difference with a normal DNS-resolution enabled scan:

[root@mmzacademy:~]nmap -p 80 8.8.8.8Starting Nmap 7.60 ( https://nmap.org ) at 2018-10-01 09:15 -03Nmap scan report for google-public-dns-a.google.com (8.8.8.8)Host is up (0.014s latency). PORT   STATE    SERVICE 80/tcp filtered http