I've had this problem for a few months now. Some, but not all, of my downloads are intolerably slow. It's not a general internet speed issue; speed tests clock in at the expected rate (1.5-2 MB/s), and surfing webpages feels normal.
The downloads in question are through websites like Keep2share, Rapidgator, Fboom, Filesmonster, etc. It used to be that having a premium account with such sites would permit full-speed downloading, but now most of the downloads are about 30-80 KB/s. But not all of them--some (maybe 10-20%) will still download at full speed. The exception is Filesmonster, where the situation is reversed: around 80-90% download at full speed, while the rest are the aforementioned 30-80 KB/s. I'm talking about premium membership here, again.
Some info that might be relevant: I use a laptop with Windows 8 and have a wireless connection with Time Warner Cable. I've tried resetting router and modem, resetting the computer, and playing with firewalls, but with no change. (But if those were the problem, you'd expect the issue to apply to all downloads rather than simply most, right?) The issue also happens on the other home computer. I haven't tried taking my laptop elsewhere for downloading, but I tried using a VPN from home, and the downloads were much faster, though still not maximum speed. (That's not an acceptable long-term solution anyway, but it might be a clue as to the problem.) I've tried different browsers and different times of day, but with no change in result. Whether a file is a slow download or a fast download doesn't seem to change: I've never seen a slow download turn fast or a fast one turn slow, and a particular file that downloads slowly will still be a slow one if I try again weeks later (same for the fast files). It also makes no difference how many or how few files I'm trying to download at once.
A lot of the standard explanations for slow downloading (ISP throttling, software bandwidth caps, busy times of day) don't seem to apply to my case, since the minority of files will still download at full speed while the slow ones crawl along.
I'm at my wits' end with this. Any advice? Thanks.