What is going on when MicroStation Visibility Calculator runs?

I have a file with no elements and one ref with some geometry (actually a dwg of 25 MB).

I create a BV and place it as separate Model in the same file.

I open ABDss4 and the file with default Model, the 3d. Takes some 30 seconds.

I then use Models dialog to open a Model with a plan BV.

I takes about 9 seconds and I get all geometry displayed on screen.

BUT  I can't do anything for more than 20 seconds more!?  

First Mesage Center says - Processing MicroStation Visibility Calculator

Then  

MicroStation Visibility Calculator took longer than expected! (23.5 seconds)

and I can start working.

What is going on and why can't I start working after the 9 seconds when I see all geometry?