In reviewing a Support Case that mirrors this Esri Community post, the customer and analyst were able to get the export to work by doing the following workflow:
- Saved the layout as a layout file
- Start a new project without a template
- Import the layout file into the new ArcGIS Pro project
- Export layout to PDF - success with minimal time
- Determination was the original *.aprx was likely corrupt.
Not an ideal workflow granted but might be worth a test to determine if this fixes the lengthy export times you're experiencing. If not, I highly recommend a call to Esri Support Services to work with an analyst.