I would suggest that you run a SQL trace while running the imports and the prepare reports tasks. To get the finest level of granularity, run the tasks one at a time.
The CS DW also has a trace tool called csdwdbgsrv.exe. This tool can be found in the directory Program Files\Common Files\Microsoft Shared\Enterprise Servers\Commerce Server. You invoke this tool by specifying a file to capture the output: 'csdwdbgsrv -f trace'. This will create a file in the current directory with trace as the prefix, a time stamp as the rest of the file name and .txt as the file extension. The output of this file might also hold a clue.