The default threshold for determining a slow link is when the connection between your client and your Domain Controller drops below 500 kbps. If you ever need to adjust this threshold number, it is possible to do so on all of your client computers. What is the mechanism for pushing this change out to your users and computers? Group Policy, of course:

User or Computer Configuration | Policies | Administrative Templates | System | Group Policy | Configure Group Policy slow link detection

Take note that this policy setting is available inside either User Configuration or Computer Configuration. If you need to adjust slow link settings for one side or the other, or both, keep that in mind when you adjust this setting.

One final note here: if you think slow-link detection is lame and want to disable it completely, configuring Group Policy Slow Link Detection = 0 (zero) will force the client to treat all connections as fast links.