Because the files needed to distribute Knoppix are so large, a number of sites around the world have volunteered to host exact copies of those files in order to "share the load" of bandwidth.
If you decide to download a Knoppix ISO file (as opposed to getting one via Bittorrent or ordering physical media), please look at the official list of mirror sites and select one of those that are geographically closest to you. Or use this page.
In the period immediately after a Knoppix update, many people will be trying to get the new files simultaneously. Also, because the process of updating mirrors is not instantaneous, some mirrors will not have the new files yet, and some mirrors will have partial files. Both of these factors make obtaining Knoppix via BitTorrent a much better option than using FTP during these times. (While "many people downloading" makes FTP slower, it makes BitTorrent faster.)