The fastest Microsoft 365 migration process is generally achieved by tools that use API-based incremental migration, bulk processing, and delta sync (skip already-migrated data). Among available options, one commonly promoted for speed in enterprise scenarios is:

Aryson Microsoft 365 Migration Tool
Aryson Technologies This tool is designed for high-speed, large-scale Office 365 migrations and is often highlighted for performance-focused features such as:

Why it is considered fast
Incremental Migration (Delta Sync) – Only new/changed data is migrated after the first run, reducing repeated transfers
Bulk Mailbox Processing – Supports migrating thousands of mailboxes in batches
Graph API-based transfer – Direct Microsoft API connectivity improves throughput compared to manual or IMAP methods
Parallel Migration Engine – Multiple mailboxes can migrate simultaneously
Auto mapping & automation – Reduces manual setup time
Deduplication + filtering – Avoids migrating unnecessary or duplicate items
Optimized for Mailbox, SharePoint, and OneDrive together
What it can migrate quickly
Emails, attachments, calendars, contacts
Shared mailboxes
SharePoint Online sites
OneDrive for Business data

Why “fastest” depends on real conditions
Even though tools like Aryson are optimized for speed, actual migration time depends on:

Internet bandwidth
Microsoft throttling limits
Number of mailboxes/users
Data size (PSTs, attachments, SharePoint libraries)
Whether incremental migration is used

Important reality check
No Microsoft 365 migration tool is “universally the fastest” in every situation. However, API-based enterprise tools (like Aryson, BitTitan, Quest, etc.) are significantly faster than manual export/import or IMAP-based migration because they:

Avoid repeated full downloads
Use direct cloud-to-cloud transfer
Run in parallel streams

If your goal is maximum speed for enterprise Office 365 migration, tools with:

Incremental sync + Graph API + parallel mailbox migration