Low WiFi speed on the ASUS laptop from the ASUS RT-AC68U router
Answer
Good day. I understand your question and problem. And I think that the router in this situation has nothing to do with it. In principle, any router will cut the speed of wi-fi, and 100 Mbps is unlikely to be squeezed out. Even through such an expensive router as ASUS RT-AC68U. And the speed of 40 Mbps on wi-fi network is not very small. Perhaps, in your conditions (signal strength, interference, etc.) this is the maximum speed at 2.4 GHz. Since you couldn’t connect your Asus G73JW laptop at 5 GHz. It’s wi-fi module just doesn’t support that frequency and 802.11ac standard.
I think if you check the wi-fi speed from some other device, even a smartphone, or tablet, it will be above 40 Mbps. Check at 2.4GHz and 5GHz. The whole problem (if it can be called that) is in the outdated wi-fi adapter of the laptop. For higher speed you need to connect to the router at 5 GHz. And for 2.4 GHz frequency and 802.11n standard this speed can be just maximum. Which is fine.
Try to connect and test the speed from some new device at 5 GHz. If the speed will be higher, you can buy a new USB wi-fi adapter for your ASUS laptop that supports 5 GHz and 802.11ac standard.