In a Client-Server environment, the Qlik performance s are the combination of 3 different times:
1 Server Engine Calculation
2 Data Transfer (between the Server and Client)
3 Client Data Rendering.
When it comes to the performance issue, the first analysis should be to understand how each component impact the total time, which should be very different from case to case.
For the Mashup, points (1) and (2) are the same, since Mashup impact only the client side. For a simple Mashup (a frame around Qlik Components) the client rendering time is exactly the same. In more sophisticated solutions you expect to have more features which should require additional time.
Hope this help