I just ran a speedtest again.... No speed difference in downstream if uploading to my provisioned 50mbit.Interesting... DOCSIS - Wikipedia I didn't know that.
The thing is, 50mbit down and 300 up is so easy for DOCSIS 3.0 (I have a 24x8 modem) that even if the modem sends one frame per direction at once, it's still your internet cap that is your bottleneck and not the capacity for the modem's inability to access the coax channels needed...
And FWIW, any speed above 50mbit where you start pushing DOCSIS 3 to the limit or needing DOCSIS 3.1 and beyond.... good luck getting a Tesla to do that, since it's a 2.4GHz 1x1 device.
At any rate, if you have 1mbit up DSL and a modem/router that does not know how to prioritize small packets in the upstream direction, yeah, uploads can make your downloads suffer. But if you're on a cable connection, that hasn't happened in sooooo many years given the way that cable modems enforce caps via traffic shaping queuing policies rather than a rigid cap.