"Log Accessed pages" Needed for Dashboard Reporting?

Quick question on Web Filtering. I noticed when I have the option "Log Accessed pages" disabled the dashboard never counts and would always show "Web Filtering is active, 0 requests served today". When its enabled however it start to count up. Is this option required to be enabled?

What type of performance hit would I expect by logging every webpage that's visited? The hardware isn't anything special, 3GB RAM/Atom D510 setup.

Thanks!