This is a older message posted by Wayne Luke at vb.com about that:
We do not have any plans to lower this value at this time. We did lower it from 60 seconds to 20 seconds not too long ago and are currently satisfied with the level of performance vs. load concerns.
You could try and edit the settings, in ACP settings/Message Searching Options/Minimum Time Between Searches set that to 0 and see if that helps you.