macOS SMB share mounts slowly only over VPN

Scenario

A share opens quickly on-premises, but the same path takes minutes to mount from a Mac over VPN.

Recommended Resolution Path

1. Test raw connectivity and DNS resolution to the file server over the tunnel.
2. Check SMB signing, dialect negotiation, and whether the VPN MTU is causing excessive retransmits.
3. Compare Finder behavior with command-line mount attempts for clearer errors.
4. If only Macs are affected, document any server-side SMB tuning needed for mixed environments.

Technician Notes

Document what changed, what confirmed the fix, and whether the issue points to a broader standards gap worth addressing for the client.