Edgerouter Lite - slow upload speeds

I have an Edgerouter lite.  I am getting slow upload speeds. 

If I jack my laptop into the network demarc, I get the full speed we are paying for : 15x15Mbps symmetrical. 

With the edgerouter lite in, I get 15Mbps  down as expected, but only about 5Mbps up. Same laptop, and jacked straight into the edgerouter. 

• I have power cycled the router. No change. 
• I reloaded the 1.8.5 firmware. No change. 
• I reset the router using the hardware reset button then reconfigured the router using the wan-2lan2 wizard. No change. 
• I downloaded a fresh copy of the 1.8.5 firmware, in case I had a bad copy. Reflashed the router, reset and reconfigured. No change. 
• I tried the 1.8.0 firmware. Flashed the router, reset and reconfigured.  No change. 

With an ASUS AC-RT87U soho router - I get the full available speed my ISP is delivering. 

Any advice?  Surely the edgerouter should be capable of delivering the full upload speed available to me?  15x15 is  not that fast. 

Any ideas?  I'm pretty new to edgerouters so the most likely explanation is that I am a total noob and have missed something screamingly obvious. But, whatever it is, I'm not smart enough to see it. 

Thanks in advance