Nikon D90 - extreme long download time

I am a new SGPro user and am practicing how to use the program.
My first setup consists of nothing more but SGPro and a Nikon D90 camera connected with a usb cable.
No other equipment is connected or configured in SGPro.
But when I run a squence consisting of two 5 sec. exposures the program locks up during the first download.
However, the image file is then already in the right directory and is readable.
At first I thought that SGPro crashed but it turned out that the whole download process took more than half an hour.
The sequence then proceeded with the next exposure with another half hour downloadtime and than ends normaly.
I can not configure what causes this odd behavior.
Mirror lockup and long exposure noise reduction are disabled and there is no SD card in the camera.
Does anybody have an idea?