Hello
I realize that when I add torrents via magnet link, KTorrent takes centuries to start the download ... Does anyone know why this happens? Is there any solution?

I have this issue too.

Opening a magnet link in the browser (Firefox) will prompt the usual 'select application'-dialog, and selecting Ktorrent is no issue.
Ktorrent itself can take up to 10 minutes to achieve any response, nor is it clear if the process is proceeding successfully or has gotten stuck.
The same problem occurs if the magnet link is pasted into the 'open URL' function of KTorrent.
If the magnet link is (successfully) converted into a traditional .torrent file, it usually opens quickly without issue.

There also seems to be a relation to this problem and opening another 'traditional torrent'; if a magnet link is opened, and goes into 'mystery state', and then another traditional torrent is opened, the dialogs for both will open at roughly the same time. I cannot say if the 2nd dialog is halted until the 1st one is shown, or if the action of opening another torrent 'kicks' KTorrent into action.

Another post (possibly on a different forum) suggested that the issue was that 'DHT needs to be turned on', but in my case, it is already active.