Honestly don't think the N810 saturates your connection or router's cpu.

Wifi is only as fast as the slowest device connected to it (sync issues.) It may be that one of the other devices has a bad connection, forcing the router to sync at slower rates. It may even be that the N810 is causing it to sync down. [Heck it may be any number of reasons - is your neighbour using your wifi without asking you? Is your ISP throttling traffic? etc, etc]

Having tried it on your sister's router - your router may just be badly designed and having issues, or your ISP may be doing naughty things. I know from personal experience that I've had no issues with the great many routers that I've connected to.

Sorry for not being much help