Dnf is blazing fast for resolving the transaction. Alas, this big speed difference wrt yum is often totally eaten up by yum using fastest mirrors. In my case I have a 100 MB connection at home, so yum usually downloads at ~8MB/s, while for dnf the speed can be just ~300kB/s. dnf should get the same kind of mirror logic as in yum-fastestmirror.
This is highly relevant, yet has to wait for the rest of the API/plugin API to be ready first. Give this some time please.
Current development version of Librepo (a library which is used by Dnf for downloading metadata and packages) has a LRO_FASTESTMIRROR which does same thing as the yum-fastestmirror: https://github.com/Tojaj/librepo/commit/acf458f29f7234d2d8d93a68391334343beae4b9
dnf-0.4.2-1.gitc1716d7.fc20 has been submitted as an update for Fedora 20. https://admin.fedoraproject.org/updates/dnf-0.4.2-1.gitc1716d7.fc20
Package dnf-0.4.2-1.gitc1716d7.fc20, librepo-1.2.0-1.fc20, hawkey-0.4.2-1.git4c51f65.fc20: * should fix your issue, * was pushed to the Fedora 20 testing repository, * should be available at your local mirror within two days. Update it with: # su -c 'yum update --enablerepo=updates-testing dnf-0.4.2-1.gitc1716d7.fc20 librepo-1.2.0-1.fc20 hawkey-0.4.2-1.git4c51f65.fc20' as soon as you are able to. Please go to the following url: https://admin.fedoraproject.org/updates/FEDORA-2013-18123/librepo-1.2.0-1.fc20,hawkey-0.4.2-1.git4c51f65.fc20,dnf-0.4.2-1.gitc1716d7.fc20 then log in and leave karma (feedback).
dnf-0.4.2-1.gitc1716d7.fc20, librepo-1.2.0-1.fc20, hawkey-0.4.2-1.git4c51f65.fc20 has been pushed to the Fedora 20 stable repository. If problems still persist, please make note of it in this bug report.