Some isp's are known to throttle back p2p networks, also if your d/ling from school they often filter or block p2p. If your isp or school is blocking you there isn't much you can do except try talking to them and see what their policies are on it or you can switch to another isp.
Then it could be in your settings. You can start by reading this thread here:
A few things to consider while configuring Limewire for optimal performance.... It will take some time and effort on your part but that's how I learned
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Also d/l speed is affected by other factors such as your connection speed, the speed of the person your d/ling from, how many sources are available, how many other people are trying to d/l the same file ect.
You can test your connection speed here:
Speed Test