I have a BT fibre connection giving 930mbps down and 110mbps up. I have attempted to use QoS to reduce the latency when uploading, but this feature does not work properly.

Firstly, it does not take effect at all unless a device is added to the high priority list.

Secondly, when QoS does take effect (device added to priority list), it results in a reduction in downstream speed when set to an appropriate speed for my connection (e.g. 900). Speed tests and real world downloads then give speeds of only around 400mbps.