In short, there a lot of factors that affect your download speed, and without getting a lot more information from you, it's hard to say which factors are coming into play in your case.

At a basic level, you can think of your download as being processed by 3 entities:
1) the peers that already have the file (or parts of it)
2) Internet routers
3) your computer

Your download cannot go any faster than the capacity of the slowest of these entities.

Congestion changes at different hours and days of the week can greatly affect the speed at which routers on the Internet can process information being sent to you. Your ISP may also change "bandwidth shaping" policies on their routers at different times of the day/days of the week.

Different hosts have different upload capacities. One day you may be downloading a file from two lightly-loaded high-capacity peers that can each upload to you at 20 kBytes/s . Some other day, you might be downloading another file from 10 hosts that can each only give you half a kByte/s.

If you're seeing a high variance in download speeds at times of low network congestion, at the same time and day of the week, my best guess is that you're downloading different files from different peers, and some of the files are coming from peers with a lot of spare upload bandwidth, while other files are coming from peers with very little spare upload bandwidth.

If you're running other network-intensive programs on your computer (or on your home network), this may be choaking your bandwidth on your end.


I used to be a computer network simulation consultant. A lot of companies spend a lot of money trying to figure out why their networks are performing badly, or else trying to decide which network upgrade options will perform the best. Large computer networks can be complicated beasts. It may require a lot of data collected from a bunch of differet computers in order to be able to tell you for sure why you're sometimes getting slow downloads.