But the topic starter @Jack asked everybody about their experience, so of course I am answering according to my experience and expectations.
On the other hand there are people who are also not that happy by the absence of smoothness in youtube and other video players.
I have it right now. 950 MiB is not a magic number, sometimes it is more (up to 2GiB probably) or even less. But it is a known (and multiple times reported) problem.
Related problem: Possible to Fix 2048MB RAM Limit for NVIDIA GPU Passthrough to HVM?
At least I have Alt+tab not working properly. It is kind of big deal. How your users overcome or work-around this? I have Alt+Tab in muscle memory and using Win10 qube is painful due to this bug:
It is not about time of the process or bandwidth at all. It is about having multiple templates (different package sets and even Fedora version and updating all them by installing something. I manage it with self-made bash scripts.
I am tracking several issues on github about this. It exists. In my case it is about disk/usb or memory IO, or CPU. Even cursor starts being choppy, reproducible half of the time. On previous and older PC with Qubes OS 3.*-4.0 I was not having this bug at all.
Please, note, that the post was made not to criticize Qubes OS, but admit that there is a room for improvement. I am also trying to solve issues and publish solutions or work-arounds when possible (including github).