आधिकारिक टिप्पणी
You can turn the light based speed adjustment off in the settings. It is there and on by default to keep you from outrunning the amount of light that is available.
Make sure you are not using any ND filters or anything like that which does nothing except make your exposure times get longer. You shouldn't be limited to such slow speed at that altitude since your GSD is pretty big at that point.