its basicly a way to tell you that your server is responding "slow".
It all depends on how hard it has it. I would recommend adding a second AD server if you have performance problems, if you dont, just change the threshold to something higher before you get an alert.
read more here:
http://technet.microsoft.com/en-us/library/cc180912.aspxif its not a problem and your services depending on the server is fast enough don't bother. just keep in mind that you might need a faster server, or an additional server at some point before you put more work on its shoulders.