Background the screenshot

[Meatballs1]

Presumably nmap would complete a lot quicker if you backgrounded the OS command with & ?

Doesn't appear to have any timeout for when it fails someone has a potential solution:
http://www.pentestgeek.com/2012/07/11/using-nmap-to-screenshot-web-services-troubleshooting/

I also found that wkhtmltoimg doesn't really handle SSL certificate problems or authentication very well so I've been looking at various tools that work better, gnome-web-photo is a bit quirky, most other command line tools are based on the same webkit/qt stuff.

I had a fudge around with PyWebShot which is based on gtkmozembed which isn't wonderful but seems to get the job done:
https://github.com/Meatballs1/PyWebShot

Main draw back is that it has to render to the actual screen, but you can get around it by using the 'xvfb-run' command to create them in virtual windows.