to what extent is the display, waterfall etc a function of the performance of the display adapter?
Often the generic pcs may have a fast processor but weak display adapter just because that is
a detail that is not marketed well except to the gamers.

Usually all the fancy features of game oriented modern GPUs are not used in an SDR program.
I don't know what happens under the curtains when you use .NET, but if you practice a safe
life and do not use it, what is usually done is to build what should be displayed in a memory
bit map, and when ready it is bit-blitted at once on the display card refresh buffer.

So, as long as the GPU is capable of a decent bit-blitting speed, it is more than adequate for
displaying spectrum/waterfall of an SDR program.

Or, if you have a GPU supported by the new libraries OpenCL of AMD, you could demand part of the
DSP processing to the highly parallel processors of the GPU. But, as far as I know, currently no
released SDR program does that.