Hi Suneer,
Why is that you are using port 8888 instead of the default 8080 port?Any specific reason?
Are you using "refresh before components are loaded" on usage tab of connection?
Try delete the existing LO connection in a copy of dashboard and create new connection and rebind the data to the same data area and try to preview. (If this works fine recreation of LO connection solves the problem)
In the copy of dashboard, Uncheck "refresh before components are loaded" to ensure it gets initialized without refreshing data and Add a simple push button and Try to trigger the data based on some push button selection and see if it still consumes huge amounts of time for refresh (if this works you can conclude if it's connection that's affecting the preview time). Since you have only one component, initializing components would not take much time.
Thanks
Mallik