I've been experiencing the exact same issue, up until the other night, i was assuming Windows 10 was causing the issue at one point, as this issue affect every single devices on my home network from connecting at proper speeds, TV, Mobile phones, Firesticks, the neighbors cat.
However, tonight i found a fix. After its being doing my head in for months.
I was find my speed was dropping to as slow as 1Mb after initially downloading at a decent speed instead of potential days to complete downloads.
Change the Port number on the program your downloading through.
If its torrents, depending on your Torrent client, go into options, then Connection, then press the Random button if available in the Listening Port box - Port used for incoming connections.
Choosing "Use different port on each startup" is a good option to enabled also as this should hopefully prevent this issue reoccurring ever again.
As soon as i did this, speeds are going back up to the higher speed as it should be and every other device connected to my BB is working perfect also.
