It appears to me that Rhino leaves the OPEN dialog box up while it reads the file. For a large file, the dialog box stays up for a long time, making the entire system effectively unusable.
This is intentional behavior. The underlying Rhino V5 program can operate on only one 3DM model at a time, so there is nothing else that would be safe to do while the file is being loaded. This will get much better in Rhino V6.
The problem is, that while your Rhino file is loading, you can’t read your email or do anything else.
I thought you meant you could not do anything with Rhino while loading a file. My mistake.
The Open file dialog delivers a file path to Rhino and then immediately closes. It does not stay open while the file is read. After getting the file path, Rhino then puts up a modeling window that initially contains just the word “Loading”. It displays the thumbnail image once that has been read, then displays the viewports once the entire model has been read. For large files, the “Loading” modeling window stays visible while the file is read, not the Open file dialog.
What you describe is not normal behavior. Two ideas. Do you have a virus scanner installed? Perhaps it has hijacked the open file dialog and insists on scanning the Rhino file before it is delivered to Rhino.
Or perhaps you have something else installed that is interfering with file opens. Create a new user account on your computer and try opening your large file again. This is a simple-minded technique that eliminates most plug-ins and add-ons that you have installed (but it typically does not defeat virus scanners).