A while back I introduced Yum Fast Downloader plugin, a plugin to use a download accelerator application (Aria2) to download yum metadata and packages (except delta rpms). The next step is trying to integrate such a functionality in Yum itself. However, I have not found the time to implement my ideas in yum, and the a few bugs in the plugin were found. Therefore, today I spend some time to identify and solve the known problems and a new update is already pushed into the plugin’s repository. Therefore, all of YFD users are encouraged to update their plugin ASAP. Hopefully, it should now work much better now. 
Thanks for your interest,
Hedayat
