Transmission is the best torrent client for Linux/Ubuntu I have ever tried. You can install Transmission using this code in a terminal window:
sudo apt-get install transmission transmission-gtk
After you're done with installing the app, you should open Transmission and change a few settings in there from the Preferences menu:
In the Torrents tab mark Stop Seeding at ratio and choose value 1.00
In the Speed tab, mark both Limit upload speed and Limit download speed and change the values in there to at least 1500kib/s for Upload and at least 5000kib/s for Download
While still in the Speed tab, go to temporary speed limits and choose similar values for both upload and download speed. Eg. 1600 kib/s Upload and 6000 kib/s Download
In the Privacy tab, mark Use Local Peer Discovery
In the Network tab, first make sure to test your port for incoming connections by pressing the Test Port button, and if all is ok, you should go to Limits under the same tab for Network and increase value for Peers to 1024 in both fields
And this should be all. Close Transmission, and next time you open it, you should see increased performance for both downloading and uploading your favorite torrents.
If you still experience issues while downloading torrent files, try to change your Download folder for torrents, default is ~/home/user/Downloads, to another folder present in your ~/home/user/ directory, for instance make a new folder and name it Torrents or whatever.
IF, by any chance, you played recently with your "Accounts and Users" settings, in Kubuntu general settings, you should make sure that your user account has full administrative privilegies and it is not restricted in any way for network access.