Interesting. I also have just managed to slow down a Far Far Away title with Command plus minus key when the title is selected. Even more interesting is that the title now follows the old rule - i.e. it always completes at the end of the title so extending the title duration slows the scrolling. It was also possible to copy and paste this title and the copied version behaved the same.
BUT
A second added Far Far Away title in the same project did not change. Neither did an added Far Far Away title in a different project. In these cases the command and minus changed the timeline scaling moving the settings slider to the left.
Then in another project which already had this title I was able to do it again.
I have a suspicion that the 2 titles which 'worked' were ones which had been added in earlier versions of iMovie but I am not sure. I have not been able to slow down titles which have been added in iMovie 10.1.
Geoff.