Difficult to answer in detail without more information.If I use the simple photo rendering V1.8 with enhanced external brightness - it slow down so much, that even a 240x123 px picture doesn't show anything (no estimated time, no draft view) after 10 minutes.
Any hints?
Enhanced external brightness takes a lot of time with a sky texture, a great image resolution, at night. The global illumination used also has an influence.
I advise you to perform a test without it being enabled to see the rendering time and have a comparison.
Enko