Some good news I think. After running speed test at DSL reports (https://dslreports.com/speedtest), I noted that the results showed a very poor score for "bufferbloat", particularly relating to uploads - the reports graded my bufferbloat as D. I had started to research alternative firmware (with no luck - VDSL routers are not well supported by DD-WRT and the like) when I thought to check the VR600 QoS settings. I had played with this before to try and ensure a good experience for my gamer son - but given up and left it disabled.
Today I thought I'd give it a try again. I know that the connection I have is limited to around 39 MBps donw and 6 MBps up so I set these in the upload and download fields. I then enabled the QoS.
The result for subsequent tests was that the buffer bloat reading was A. Result. I thought I would share this here so others might try it out.
There is very little documentation of the TP-Link routers' implemention of QoS beyond what is seen in the UI and "how to" documentation that just says how to turn it on or off. I would love to know if the VR600 and its kin support any form of Smart Queue Management (SQM) and if enabling QoS enables SQM.
I will observe to see whether there is any noticable effect on my experience and whether speed test results continue to be better than before - and report back here.
* Read more about bufferbloat at www.bufferbloat.net