SH 3.0 is poor, I have an ongoing fault ticket for SNR and FECS (F005039426) and usually I only get 20-40Mbps on Xbox. Dont really see the point in having 152Mbps when I dont see those speeds when on Xbox.
Whilst my download and upload speed on the Superhub 3 was what I expected, I had latency issues. The problem that I had is described by the Register news paper: https://www.theregister.co.uk/2016/12/03/intel_puma_chipset_firmware_fix/. The problem was resolved when I switched back to the Superhub 2ac. Your problem seems to be download speed, so I don't know whether switching routers may be helpful. Have you considered asking for credit on your account whilst they resolve the issue?