WAX 206 Ethernet reduces speed by 90% on large file transfer

Using a WAX 206 as an access point. Firmware is latest  1.0.4.0.

I have noticed, for large file transfers (2 GB plus) that initial speed is as high as could be expected - about 112MB/s (about 900Mb/s). However, after a variable period to time, 10 seconds plus, the speed drops to a tenth of that - about 12MB/s. Changing the ethernet port does not fix this. If I move the target laptop to another location and connect it direct to the router, there is no problem - the whole transfer completes at 112MB/s. If I use a USB ethernet adapter connected to the WAX 206 (i.e.  a different NIC), the connection drops speed in the same way. Using WiFi, the initial speed is about 71MB/s and drops in the same way but to about 20MB/s.

The drop in speed is not, therefore, due to the NIC or any other part of the hardware in the laptop. It is due to the WAX 206.

Anyone seen anything like this? Is there a fix?