If latency is going up during large file uploads, you can measure it using this tool:
https://www.waveform.com/tools/bufferbloat

Then if you apply some queueing changes at your router side, you can quickly measure whether it's made an improvement or not.

Excellent - thank you. Much appreciated.

Thank you - that's a great tool.

I've been "lucky" enough to be able to test the upload theory. I've been uploading 250 GB onto a new dropbox account.

As you'd expect, the latency has gone up, but not much at all.

https://dl.dropboxusercontent.com/scl/fi/flyg4o3o2fb...

That looks fine - is this with the 95% applied or as it was? In any case, no issues there really.

So far as it was. I'm very pleased with what I'm getting.

Thanks also for the advice.