Downloads have built-in "checksums" (I think that's the correct term that basically check to make sure the bits of data are being downloaded correctly.
Sometimes the previous check has to redownload the previous bits. That can cause the time to go up.
Also, it may be that your bandwidth speed is slowing down or lots of others are accessing the bandwidth.
BUT if the download goes way, way up—like instead of 12 minutes, it goes to 6 hours, something's wrong with the installation.