FreeFileSync Slow Network Sync: Causes and Fixes
Diagnose slow FreeFileSync transfers over SMB, NAS, Wi-Fi, and Ethernet without risking synchronized data.
Quick answer
Measure a normal Windows file copy over the same path before changing FreeFileSync settings. If the baseline is slow, investigate the network, NAS, VPN, storage, or Wi-Fi first.
What to check first
Thousands of small files can transfer much more slowly than one large file of the same total size because each item creates filesystem and metadata work.
Windows-specific factors
Time-and-size comparison is usually faster than full content comparison. Use content comparison only when timestamps cannot be trusted and the extra I/O is justified.
Safer troubleshooting workflow
Review Windows Security, destination free space, drive health, network stability, and the FreeFileSync log. Do not disable protection globally to chase speed.
How to verify the result
After tuning, run Compare again and verify source, destination, filters, and deletion actions before synchronizing important data.
Frequently asked questions
Should I change sync mode to fix this?
Not until the underlying cause is identified. First verify paths, permissions, comparison results, and logs.
Is a successful sync the same as a verified backup?
No. Important data should be checked at the destination and periodically restored to a separate location.
Related: all FreeFileSync Windows guides