You'll learn more about networking from using Nmap than from looking at the source. Always use -v and maybe even -d, then investigate what the output is telling you. And read the book: https://nmap.org/book/
Not aware of this issue. Please file a bug report at http://issues.nmap.org/new or dev@nmap.org, thanks!
Your second command is using TCP Connect scan (-sT) instead of SYN scan (-sS). Not sure why it would be doing this, but add -sS and see if speed difference is better.
You'll learn more about networking from using Nmap than from looking at the source. Always use -v and maybe even -d, then investigate what the output is telling you. And read the book: https://nmap.org/book/
Not aware of this issue. Please file a bug report at http://issues.nmap.org/new or dev@nmap.org, thanks!
Your second command is using TCP Connect scan (-sT) instead of SYN scan (-sS). Not sure why it would be doing this, but add -sS and see if speed difference is better.