(Sorry for the delayed response! Most people ask questions in our blog...)
Your "start" time includes everything up until the plugins are fully loaded. So that means plugin load time could easily be the culprit.
347ms before plugins start to load is a bit long but perhaps not so bad on your server. It all depends...
However: 762ms for Posts selection is rather a lot. Do you have a ton of posts by any chance? If so, you may need to do some careful optimization, as Chaos Inc suggested :)
To examine this in more detail requires some WP code changes that can't be done automagically by WP Tuner. For further hints, look at the wptuner page, in the "other notes" area. I'll try to link: here