Don't remember the user or the name of the thread, but one I read, said to check your connection first.
As for myself, I have a 1500/400 kbit/second. This means you divide 1500 by 8 to get the maximum KiloBYTE/sec theoretically available. (Other users, programs, distance from ISP's connection box, etc, will diminish this) Using myself, I will get 187,5 KB/sec maximum.

In Tools > Options > Download you can then adjust your settings according to your theoretic max. It's advised to take the speed you get and subtract 5 from the end result to begin with. Then start with 4 simultaneous downloads. If it's working and you don't reach max without any of them being severly slow all the time, increase by one until you find what's right for you.

Same goes for upload. Don't go all the way to max on the speed, but slightly under. Here it's probably good to start with just one download. If that one works OK without reaching max, adjust by one more if you wish. Continue until you find your right setting.

The "right" setting may, depending on time of day, other people using your line, your ISP, etc, be off somewhat. Just adjust like previously described, and you will be fine.

I am running 8 simultaneous downloads, never going below 90 KB/s except for when a file is finished and it connects to another host for next file. And my current uploads is 5. This however never uses the full bandwidth I have assigned to it.