If the backup file in question is a native backup, then the program has to seek all the way to the end of the file to find the "header" information. I realize that sounds a bit tail-about-head but MS has to do this for compatibility with tape drives.
SQL Backup files put the header at the front, so may work faster.
Either way, though, this operation should take minutes rather than hours.
I'm also aware that there is a search for the first page containing the schema, and that can be time-consuming.
I suppose the only way to work out what this software is actually doing is to debug it.