I just got gigabit internet the other day from 350Mbps, and when I do a speed test from my PC that's connected via Ethernet I only get 750Mbps. If I do it straight from the modem (which I have in bridge mode) I get around 950Mbps, but through my router my connection drops to 750Mbps.
My router has a built-in speed test, and through there I get around 850Mbps. I read online that turning QoS and other features off might help, and I looked around and set QoS to low (there is no off option from what I can see), and all other settings looks normal from what I can tell.
I am using Cat5e cables, and the router model is TP-Link Archer C2300 v1.0
The test highlighted in red is when I had my PC connected to my modem directly, the ones in the 800's is straight from my router, and the ones in the 700's is from my PC to my router.