Hello and welcome!
Do I understand correctly that this error is repeated in the IDE all the time (not a temporary connection problem or arduino.cc server availability)?
I can’t reproduce it even with “modem connection” speed. 
Maybe you’re using VPN / Proxy?
Manual installing
All toolchains and hardware configurations on Windows machines installed into:
C:/Users/%USERNAME%/AppData/Roaming/xod-client-electron/packages/arduino/
This one (avr downloaded from Arduino.cc) will be installed into subfolder hardware/avr. Check the full path to contents: C:/Users/%USERNAME%/AppData/Roaming/xod-client-electron/packages/arduino/avr/boards.txt
If you’ll have the same problems with downloading toolchains:
Tools should be installed in tools subfolder (and downloaded from our Google Cloud Storage).
Here is a screenshot from MacOS for clarity, the structure of packages folder is equal on all OS.
I hope I help you.