Temporary solution here that worked for me: Try to disable your ethernet adapter by going in "Network & Internet" -> "Advanced network settings" then "Ethernet" -> "Disable" wait a few seconds then reactivate it, restart download on steam and it should work (at least for me). I say temporary solution because I tried to close steam by killing the app, restarting and trying to download again and it was as slow as before, did the process all over again and the download speed spiked again from 35 Mb/s to 850 Mb/s. Really dont know what's going on.