Both the wort and the water need to flow fast enough that you achieve turbulent flow in both "sides" of the heat exchanger (Reynolds number > 2900). If either is slow enough that you have laminar flow (Re < 2300), then it will take a very long time to chill, e.g., because the wort along the inside of the tube will chill, but won't mix with the hot wort in the middle. If your cooling water is flowing fast already, I would guess that you need to turn the wort flow rate UP so that you achieve turbulent flow in the wort.
That chiller has an 11.9mm i.d. wort tube. According to my spreadsheet, if you are taking more than about 5 1/2 minutes to pump 20L of 1.065 wort through that tube, you will have fully laminar flow, which almost certainly will not result in adequate chilling. If you pump that same 20L through in 4 minutes or less, you will have fully turbulent flow, which will vastly improve your heat exchange. Of course, if the surface area of the heat exchanger is too small or doesn't have enough thermal conductivity, then you have a different problem. But since others have had good luck with that chiller, my bet is on laminar flow.
thats not what i'm trying to say. The coolant water should be unrestricted. the wort has to be slowed down, as you said for more contact time. the valve to restrict it should be on the outlet end of the chiller. it might work both ways but its better on the outlet to let full wort volume flow into the chiller and slow its exit. i think there is better contact and heat removal inside the chiller.
OK so I mis understood when you said no one should restrict wort flow into the chiller. Well in my case, and probably a lot of others. the valve to regulate wort flow is on the outlet of the pump. So thats where wort flow regulation occurs. Placing a valve on the outlet of the chiller would result in the same effect. but an unnecessary addition to a system that already has the means to regulate flow with a valve on the pump outlet. If anything I would agree that no one should restrict the inlet to a pump.