Writing a good presentation is a process that takes time. There is the time to complete the analysis and make the graphs and tables, but also time to ponder the story and the flow.
If you somehow end up rewriting your whole deck in 30 minutes the day before the presentation, and leaving out many of the data charts that took you days to make, then it does not mean that you made mistakes, got it wrong, or wasted your time. You actually had to go through the whole loop in order to pull off that 30 minute rewrite.