Throughput on DMZ port without UTM

Hi
My Internet connection has a raw download speed of 1 Gbit/s. When I run a speed test directly behind my modem without going through the firewall, I get a DL speed of around 870-930 Mbits/sec.
If I run the same test connecting on port 7 of my USG110, where I have a security policy rule with no UTM service at all, the speed drops to around 300 Mbits/sec. 
Shouldn't I expect a higher speed there? Why does my USG slow down the speed so much even if I have not configured any UTM service on that port?