I assume that everything else works, AP are connected to the ZD, clients can connect and pass traffic on the correct VLAN and so on and that the only error is the packet drop when running speedflex, correct?
This can be a side effect of how speedflex works. When you run speedflex, you create two endpoint that will be exchanging traffic and reporting the results to another system. If you run speedflex between a ZD and an AP and both are connected to the same subnet and they both are on gig connection, you will not see any packet loss, but if the AP is connected to a 100 Mbps instead of a gig like the ZD then the downlink (ZD to AP) will experience packet loss where the uplink will not. This is because when you send UDP traffic form the ZD you are sending it at a gig rate and the AP is only able to receive at 100 Mbps. Not all the packet at a rate of 600 Mbps from the ZD will be able to be recieved by the AP, so you will see packet loss on the downlink. When the donwlink is done the AP will then start send traffic back to the ZD, but since it's connected at 100 Mbps it will only send packet at about 90 Mbps which can be easily received at the ZD's gig connection easily so you do not see packet loss.
You might consider using ZAP instead of speedflex and running at a rate at the lowest common denominator, -r 10 for example since you have a 10 Mbps link between the two testing points and use the TCP (-t) option instead of the default UDP.