6 hours does look like an extremely long time to create the undo file, even if the entire 5.5 GB contains uncommitted transactions. Can you compare the restore speed against the other server? You can check in msdb..restorehistory for the time the restore started, then look in the SQL Server log for the time the restore completed.
SQL BAK Explorer - read SQL Server backup file details without SQL Server.
Supports backup files created with SQL Server 2005 up to SQL Server 2017.