Permalinks: site.com/post-tite vs site.com/post-id/post-title for SEO and Speed

When 3.3 rolls out (scheduled for later this month), the issues with /%postname%/
have been fixed. The performance issue shouldn’t be part of the factor. If you prefer /%postname%/use it.

Link:
http://core.trac.wordpress.org/ticket/16687