This is by design. LimeWire slows down slightly when it's not the foremost application so that other programs can make use of the internet. Prior to doing this, we received many more complaints that using LimeWire made browsing the internet impossible. Unfortunately, there's no way to figure out if another program wants access to the internet. If there was, we could make it so that LimeWire would only slow down if another program was actually using the internet.