When consuming a large number of SharePoint list items, the Design View experience in SharePoint Designer may become slow and unresponsive.
This happens when there is too much data being returned to the client. The "what you see is what you get" experience is not designed to work with massive data sets.
There are a couple of workarounds. One is to use Sample Data and the second is to return less data to your view by creating a temporary data source filter.
To use the Sample Data functionality please refer to the links below:
To create a filter at the data source level for a SharePoint List please refer to the following articles:
This article covers Copy and Modify functionality which helps create a filtered data source: http://office.microsoft.com/en-us/sharepoint-designer-help/copy-and-modify-a-data-source-HA010119471.aspx
This article covers the concept of filtering: http://office.microsoft.com/en-us/sharepoint-designer-help/filter-data-in-a-data-view-HA010103041.aspx