I have downloaded a lot of Ubuntu .iso images over the years and every time I downloaded an Ubuntu .iso image it maxed out all the available capacity from my internet service provider, except for the single time I tried downloading the Ubuntu .iso file from a torrent mirror site which was a lot slower. Your mileage may vary, so if you don't want to waste your time waiting for the Ubuntu .iso to finish downloading you should try downloading it with both http and torrent, and cancel the download that is slower.
btw I always download all Ubuntu iso images from the official Ubuntu website and I use a download tool that has resume interrupted download functionality like wget has using wget -c file.iso
or GNU Wget for Windows, so I don't have to download the same bits twice.
-c
--continue
Continue getting a partially-downloaded file. This is useful when
you want to finish up a download started by a previous instance of
wget or by another program.
An alternative to wget that works on all platforms (Windows/Mac/Linux) is the popular DownloadThemAll extension for Firefox. DownloadThemAll will help you select, queue, sort and run your downloads faster. It comes with advanced ways to select what links to download, and will remember your previous decisions so that you can queue more downloads with just one click. DownloadThemAll can also automatically or manually restart interrupted downloads in the same place where they were interrupted so you don't lose the part of the file that you already downloaded.