How is it measured?
Offline Files measures the speed of a link based on the packet latency between the client and the target server.
This is reported by the network driver and TCP stack and is then used to compute the throughput to the server for comparison
with the configured throughput and/or latency policy configuration.
Windows Vista SP2 further improves this feature by taking into account both the inbound and outbound traffic.
Out of those two, values it takes the smaller of them when determining if a transition to slow link is needed.
You can configure the Slow Link in the GPO:
Computer Configuration\Administrative Templates\Network\Offline Files\Configure slow-link mode
On client computers running Windows 7 or Windows Server 2008 R2,
a shared folder automatically transitions to the slow-link mode if the round-trip latency of the network is greater than 80 milliseconds, or as configured by this policy.
After transitioning a folder to the slow-link mode, Offline Files synchronizes the user's files in the background at regular intervals, or as configured by the 'Configure Background Sync' policy.
While in slow-link mode, Windows periodically (every 2 Minutes) checks the connection to the folder and brings the folder back online if network speeds improve.
If you do not configure this policy setting, computers running Windows Vista or Windows Server 2008 will not transition a shared folder to the slow-link mode.
Computers running Windows 7 or Windows Server 2008 R2 will use the default latency value of 80 milliseconds when transitioning a folder to the slow-link mode.
To prevent computers running Windows 7 or Windows Server 2008 R2 from using the slow-link mode, disable this policy.