If you observe drastically reduced download speeds (such as anything over 5 minutes), it is most commonly due to issues translating the date fields between the CU*BASE file and the PC file. The Data Transfer tool will still complete the download, however, there are steps you can take to speed things along. 

To troubleshoot this issue, access Tool #1375 File Transfers (Upload or Download), navigate to the From IBM i -2 tab and select the "Format Options" button in the File section. 

On the pop-up window that appears, change the date format from "[MDY] Month Day Year" to instead say "[USA] USA Standard" (this will display dates as MM/DD/YYY in your final file) or "[ISO] International Standards Org" (this is will display dates as YYY-MM-DD in your file).  Click "Ok" and proceed with the original download window options as normal.