Welcome Alexander 
The timeline’s length in seconds can be adjusted in Bongo’s Document Properties. Default is 10 sec.
Obviously in Preview the number of frames/sec Bongo can make is limited by the speed of the CPU of your computer and influenced by the complexity of the model and the display mode of the viewport(s).
The ‘Redraw only active view’ option (to be found in the Rhino’s Bongo Options, and by default set ‘off’) can help to shorten the redraw-process (when several windows are open) and hence smoothen the preview.
