I just tried some other Proton related repositories and other projects. With tkg it’s the same (binary archives downloading slow, source code downloading fast). After that I just now tried downloading the same projects via remote desktop at my workplace (Windows) and it works without any problems. Also (as mentioned), my notebook with EOS had the same problems in another household with a different internet connection.
So for now it seems like Linux is the main difference in why the download isn’t working properly.