The fastest way to convert large OST files to PST is by using a professional standalone converter instead of relying on Outlook's manual export option. Outlook can be slow when handling large mailbox files, and the process may freeze or fail if the file size is very big. A tool like the SysInfo OST to PST Converter Tool is designed to process large OST files directly without depending on Outlook performance.

The software scans the OST file efficiently, loads all mailbox items (emails, contacts, calendars, tasks, and attachments), and converts them into PST format while preserving the original folder hierarchy. It also provides options like splitting large PST files into smaller parts, which improves performance and prevents corruption. This makes the overall process faster, smoother, and more reliable for large data files.