When opening DGN files to a Drawing Model the process is very slow, often taking 5-8 minutes. We've noticed that the message "Processing MicroStation Visibility Calculator took longer than expected" is usually displayed in the Message Center, though we're not sure what that means.
Choose Building Designer > Enable Auto Annotations and uncheck this option. By disabling it, you should be able to alleviate the Visibility Calculator portion of the load time which is often the largest chunk.
This option controls a process that scans drawings to determine the current element display state, so that drawing annotation is automatically generated when the display of an annotatable component has changed. For example, I start with a Drawing Model that includes three doors, each with an ID symbol. I then go back to my 3D model and add a fourth door that will be visible in the Drawing. When I next open that Drawing Model with the Enable Auto Annotations option disabled, the annotation for that fourth door will not automatically display since the scan process has not taken place. Once re-enabled, simply reopening the Drawing Model will display all annotation changes, including that fourth door.
What you can do for increased performance is to leave this option disabled for day-to-day work, and enable it only when you need annotation to be updated. Keeping in mind, of course, that this only applies to things that have changed - any existing annotation will remain whether the option is enabled or disabled.
You can turn fully automatic annotations off by setting BB_AUTOANNOTATION=0, then keyin "bbannotate byvisibility" when needed. Please see Drawing Rules, Auto Annotation and performance for further details of this approach.