If you are on a Mac computer using the OS X Mavericks (10.9) operating system, a new 'feature' called APP NAP forces applications operating in the background to go to 'sleep', for various reasons.
If this 'feature' is affecting how your FrostWire works, you would see the download and upload speeds pretty high when the FrostWire screen is active, then when you minimize the app, the download speeds would go drastically down. When you maximize FrostWire again, the speeds immediately go up.
To turn App Nap off for FrostWire, to to your Applications folder, then right click on the FrostWire icon and select More info. You will have an option to check 'Prevent App Nap'.
This will work only for FrostWire. If you want to turn it off entirely, you can go to your Preferences > Energy Saver and change your settings there.