This bug is present since some time, but I'm not sure when was it introduced. The problem is if you select a text area with the mouse in a form (like this box I'm editing), Konqueror stops responding for a few seconds before you can go on (delete the text, or type some text instead of it). I don't see increased CPU usage during this freeze.
Looks like bug #77237, which is already closed as WORKSFORME, altough it's still present.
Ok, Qt patch #48 solves the problem... *** This bug has been marked as a duplicate of 80072 ***