It is one topic of debate quite a long time. With testing out few different custom structure. i came to know this:

custom structure with sitename.com/%postname% is good for SEO BUT ive got better SERP rankings with Post ID or any unique id (number) in the url.

The default sitename.com/yyyy/mm/dd/%postname% is itself better performance wise.

When i used
sitename.com/%postname%/%postid%

The numeric part in the last it showed better...