I've had this issue on and off for the past 2 years or so. I also have it on Youtube. If I start downloading a video, it will throttle down to 10-20 kB/s over the course of a minute. However if I start downloading a video but don't choose a destination, just leave it at that window, the video will be downloading in the background at full capacity. I'll then monitor the network usage in Network tab in Windows Task Manager. When it's done downloading I'll choose the desired destination and wait while the file transfers there locally.