The Intel® 10 Gigabit XF SR Server Adapter is capable of x8, but will work in a slot that is mechanically x8 and electrically x4. If you are negotiating 4 lanes on the bus, then your system is limiting the adapter to 4 lanes.
Depending on how you set up your speed test, you might be encountering other bottlenecks. Take a look at Network adapter throughput performance considerations for starters. I hope this helps.