Graphical Message Flow viewer uses the same components to fetch message flow data as BizTalk administrator console. There is no pre-fetch of data. Data for message flow is pulled as you click the message points in the Graphical Message Flow Viewer. There is no performance impact for your existing BizTalk applications.
Graphical Message Flow demands data as and when required, for instance when you prefer to see the context and content of a message the data is fetched when you click on a message.