I know i am not the person who cares the Performance internally, But recently i came to know these?
When we upgrade to any recent version off course, There might be come advantages and disadvantages. I have gone this earlier. I got some changes and i clear to know those. I would simply ask you to read each and every point with Architecture and agenda by help of sticky paper to know the engine of Qlik products (QIX - Engine) Migration questions : Qlikview 11.0 SR2 to 12.1 SR3
Until now I have no own experience with QV 12. But AFAIK there are only slight changes by the engine from a performance point of view (mostly it was a harmonizing with the Sense engine) so that I wouldn't expect significantly performance differences between 11 and 12 within the most environments unless it's a multi cluster environment which should get some improvements (but this is just from hearsay).
I have seen a very good presentation from hic to the QIX engine by the qlik dev group meeting in Hamburg last year but don't remember it well enough in the details. Maybe Henric could give you an answer here.