Your download speed will depend on many things - how many hosts you're downloading from, their upload speed, how many upload slots they have to share that bandwidth, your own download speed, how many files you're downloading at once, whether your ISP is throttling downloads etc etc.

Have a look at this thread (mainly the first few posts by Murasame) & see whether any of the tips help: A few things to consider while configuring Limewire for optimal performance...