pre-scanning for vulnerability scans?

[offset <offset () core svcroot net>]

Greetings,

My goal is to determine ways to speed up network vulnerability scans on a number of /20 networks (but not at the 
expense of accuracy)

Given the goal above, anyone have experience testing accuracy and speed for host detection and full port scans using 
various network scanners (ie, unicorn versus nmap)?

Do you find that bandwidth is the limiting factor to negate differences between scanners?  Assuming source is typical 
broadband (dsl, cable) around 1Mbps upload speed.

Looking to find most efficient methods of the following, assume stealth is not the goal, but accuracy is
1. host up detection (detecting ports (ie, 80, 443)), mark for followup later (queue for full scan)
2. full port SYN scan on detected hosts (TCP only)
3. vulnerability analysis based on host/port information

Regards

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