Clips speed/duration changes are not yet implemented: https://bugzilla.gnome.org/show_bug.cgi?id=593828

Crossfading: see the user manual :) however, there's bugs with it currently, so it doesn't work properly:
https://bugzilla.gnome.org/show_bug.cgi?id=632414
https://bugzilla.gnome.org/show_bug.cgi?id=613767