Overbooking is something that is done by your ISP. It means that if you have a 1Mb lease line with overbooking of 1:4, you get 1Mb when available but only 256Kb (1/4th) is guaranteed. Unless you checked with your contract or ISP servicedesk, there is no way to find out what your overbooking is on that particular line.
Again, can you do some tests with another machine (like just a normal pc or laptop) and see if you STILL have that slow internet?
Either way, I advice to contact your ISP and let them do line tests. If you have a leased 1Mb line, you may expect to get 1Mb and not 600Kb....
Btw... do you have the monitor module installed on your Zentyal machine. It can give some information on line speeds and system resources.
Bottomline: you have to figure out if it is Zentyal or your connection that is causing the slow internet connection.