The UpdatePanel is a workaround technology and unfortunately there is now way around this limit.
Usually the ViewState is the biggest part of the POST data so you can try disabling the ViewState for the whole control.
However, it is reasonable to keep the timeline size decent because it affects the Scheduler performance. Having 9600 time cells in the timeline and CellWidth of 20 pixels would result in a grid 192,000 pixels wide. That is not going to work well.