Your iTunes is probably set to download 3 files at a time from the iTunes store. So if you sign up for say a podcast that has 25 items and you queue all 25 to download at a time it will immediately start downloading three of the files and when one finishes it will immediately start downloading another so there are always 3 files downloading until it has made it all the way through the list. This can really bog down your computer depending on how much power your PC has.
To resolve this problem I was looking for a setting in the Options menu to set the amount of files to download at once to one or possibly two however I was unable to find this setting. The other thing I was looking for was a way to rate limit the amount of traffic used while downloading files or to provide some type of application level QoS (Quality of Service). I was also unable to find this setting.