Allegedly, on or about 13 November 2015, Porfirio Andres Paiz Carrasco sent:
To add it: su -c "echo 'keepcache=true' >> /etc/dnf/dnf.conf" su -c "echo 'deltarpm=true' >> /etc/dnf/dnf.conf" su -c "echo 'fastestmirror=true' >> /etc/dnf/dnf.conf"
I must confess that even adding this to my dnf.conf file sometimes the problem back or appear.
Asking the obvious question: Have you looked at the dnf.conf file, rather than just append things to it?