Issue: Downloading from the repository is (very) slow
Solution: Wait, manually select a mirror or make your own mirror.
Kali-Linux has multiple different mirrors around the globe, and it will automatically select the one which is closest to you based on your GeoIP. From time to time, certain mirrors may become slow for any number of reasons. You can either wait a few hours and see if the speed increases, else you can manually select a mirror.
Click on both 'mirrorlist' link below and find select a mirror that suits you. You will then need to edit your /etc/apt/sources.list file accordingly with the chosen values (don't forget to run 'apt-get update' afterwards).