The nbg6817 is basically the same hardware as your r7800, so the same background applies:
While I would expect a (wired) throughput of around ~550-650 MBit/s (well, you don't say anything about your WAN configuration, if you need PPPoE, ~300 MBit/s might be reasonable), it won't do anything close to 1 GBit/s.
The WiFi performance is a bit harder to judge, without knowing a lot more about your environment (congested environments with many concurrent neighbouring APs will produce more interference -the airtime simply being exhausted- than rural areas where you might not even see your neighbours' houses), nor do you tell anything about the test distance or the clients used for these tests. Most clients (notebooks, phones) will be 2x2 at best, providing you with a (theoretical, not achievable in practice) link rate of 866 MBit/s - under these circumstances, the best you will get out of it in practice will be around ~300-350 (400, if you're very lucky, but neither would ~250 be anything to complain about with lower end clients) MBit/s. 75 MBit/s sounds a bit as if your client would opt to use the 2.4 GHz band, for which there may be several reasons (keep in mind that 5 GHz will take longer to come/ get found by clients, clients may be sticky, etc. pp.).
Always check wired(-only) performance first, if you're in doubt about your router's performance or potential issues with your WAN connection.
At least for the initial WiFi performance testing, avoid complex (multi-vAP) configurations, while those should not cause this amount of throughput loss, there will be some - and the additional interfaces will also delay the interface coming up as well (sticky clients).