@jonathan-zuntz maybe also, ensure no other resident program or tools or window are using internet. Even other browser's window, music players, chat or videos, sync apps, advertising videos on browser windows, anything. You could close all browser tabs then reboot system, then close all tray icons and any resident programs, then retry the download. Before to begin download, check the band that the browser can access, for instance on speedof.me site.
If after all this is not enough, maybe you should optimize registry settings for internet, but don't use third tools, only use Siv (system Info viewer by Ray Hincliffe) and in menu Tune - Windows, go to page User tuning (increase a bit the "max connections" values - normal and max - if they are 1 or too low under the max limit). If this is not working enough again, there are other settings but they are slight dangerous, easy to go worst than better, they require study.
Also Microsoft tool to solve connection problems, could help well.
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