Very poor internet performance - 53% plan speed

I subscribe to 1.2GB internet plan and noticed that during peak times, say after 6pm evenings, I get terrible performance, ranging anywhere from 60 to 79%. Yesterday evening it was 53%.

Let's be clear, this is not a WiFi issue, this is running "Test Internet Connection" in the Xfinity app.

This is also not a one off issue, I've been measuring performance over the past few weeks and there is a clear pattern;

During peak times 6pm-11pm I observe the above stated performance. During off peak times, such as 6am, I usually get  1.3GB - 1.4GB. Often times even mid afternoon, I get over the plan speed.

To me this is a clear indication of over provisioning. I fail to see how this could possibly be either an equipment or on-premise issue. Nothing changes between the performance readings - No reboot, no re-positioning of equipment, no tightening of cables, everything remains as is between readings.  Performance readings provided by Xfinity's "Test Internet Connection" in the Xfinity App and confirmed by various other peed tests (additional testing performed with wired connection to Xfinity router/Modem).

Just to confirm that performing the Gateway restart does NOT fix the issue. After restarting the Gateway, at peak times, the performance remains the same. This is with only Xfinity hardware (XB8)

Can anyone shed any light on this? is this typical of Xfinity network performance? Does Xfinity routinely over provision services?