nmap is a bit of a joke to be fair, it is incredibly slow and there's no amount of fiddling with the settings that'll make it faster (even for simple SYN scans).

Scanning every single TCP port on a host will require 65536 packets, those packets will all be about 40 bytes in size. That's only 2.6 megabytes, so on a 100Mbit link that should take around 200ms to send.

This is a pretty trivial exercise to implement yourself, but for pre-existing tools you can look at masscan and zmap. However, these will not provide the version detection nmap does.