In general you should reduce the time window to what your dispatcher can realistically view and dispatch on the screen.  Don’t set up the query to pull in a lot of data that is not usually used by the dispatcher and can only be viewed by scrolling far to the right on the timeline.  Use the calendar control to switch the window to a new time period.   When you say one particular site is having the issue, what is different about that site? Does it a much larger team than the others?  Do you have custom fields displayed that maybe need to indexed?