I'm only getting 10Mbps through my new XG Home firewall

Question

I'm getting gigabit Internet in a couple of weeks. I had been using a small lab firewall from work at home that's only good for 50Mbps with threat prevention enabled. So I researched Sophos UTM hardware in their appliances for gigabit systems, and purchased a new PC (components I put together myself): 
 
 Intel Core i5-6500 (3.2GHz, quad-core) 
 16GB RAM 
 128GB SSD drive 
 2 identical gigabit NICs 
 
 I was not able to get UTM to install, so I found XG, and installed that, which went smoothly. I was very happy with XG, until now. 
 Currently I'm on U-verse with their fastest speed. When connected directly into the U-verse router, speedtest.net shows 80Mbps download. When I connect to a gigabit switch connected to the LAN interface of my new XG firewall, the best I get is 9.5Mbps. This is with or without the threat protection features enabled. 
 What the heck is going on? I find it hard to believe that with the specs I have on this computer, that I can't do any better than 10Mbps. Am I missing something really obvious? I haven't touched QoS.

Chibana · Accepted Answer

They're RealTek, but your question got me looking at the NIC settings. I don't know why I didn't think to look at that sooner. Both NICs were showing 10Mbps auto/auto. I don't know why they autonegotiated to only 10. I set them both manually to 1000 and now my speeds are normal again. I went back to auto/auto, and they both show 1000 now. Strange. I'll leave them hardcoded to 1000 just to be safe. 
 
 Thanks for the replies! I'm glad I don't have to panic and find another solution. I'm still a bit worried that this firewall won't keep up with a gigabit Internet line, but we'll see.