The default USB Audio Codec driver used by the ZED series (apart from the ZED-R16) does not allow you to reduce the latency by lowering buffer sizes.
If you are using your ZED in an application where you need low latency monitoring then you can install the free ASIO4ALL universal WDM driver which will allow you to run your ZED at much lower latencies, the exact amount will depend on the capabilities of your system.