Second, can you detailed a bit your thought (above)? What is better on SDS800x than in DHO800?
First would be the partially faulty windows of the FFT function, here a user had proven that they could be easily corrected, but actually this is Rigol's job to do this.
Secondly, the Rigol, like all Rigol models, lacks useful additional functions such as average and peak detect.
At least average should be "mandatory".
This would also be easy to solve if rigol were to attach importance to it.
Let's get back to your main concern, the measurement of "speed", the FPS.
I still don't quite know what you mean by that, as you mentioned realtime spectrum analyzers as a comparison.
This is a different technique behind it, the FFT function in a scope is, as the name suggests, a function, calculated mathematically.
This cannot be done/displayed in real time per se.
The "speed" then stands and falls with the number of points, the time base and the memory.
Just as an example, if the time base is set to 100ms/div in order to keep the RBW small, for example, an acquisition takes at least 1 second.
Then there is the calculation and finally the display of the function, i.e. either way you would be miles away from your 60fps.
If I were you, I wouldn't get stuck on that, instead I would focus on other qualities.
Nevertheless, I can make an FFT according to your specifications.