Server could have just been overloaded at the time. Not sure.
For future reference, if a download is slow, you can cancel it and try one of these directly to download the offline package:
https://get.geo.opera.com/ftp/pub/opera_gx/
https://www.opera.com/download#opera-gx
https://download1.operacdn.com/
https://download2.operacdn.com/
https://download3.operacdn.com/
https://download4.operacdn.com/
https://download5.operacdn.com/
https://ftp.opera.com/pub/opera_gx/