Few examples of how quickly PunkBuster can detect cheaters:
Bad Company 2 - Day 1 (MD5 Tool)
http://www.pbbans.com/mbi-viewban-606fb0e9-vb161738.html
Bad Company 2 - Day 3 (VIOLATION (GAMEHACK) 80474)
http://www.pbbans.com/mbi-viewban-4a17e4f2-vb161966.html
Medal of Honor - Day 2 (VIOLATION (AIMBOT) 50120)
http://www.pbbans.com/mbi-viewban-e4e635fc-vb192444.html
Actually Tripwire may have set a new standard depending on how well PB and VAC works together for RO2. If people canceled pre-orders over PB then that's their loss. Pointless move when you have the option to disable PB.
Bottom line both PB and VAC can be slow when banning cheaters but it's the admin tools such as PBSS, CVAR checks, MD5 Checks, streaming ability etc etc that makes PB a better solution.