It would be great if the guys from Rigol had a look at a venerable, ancient Lecroy 9400
(*)
Even though the slow M68000 CPU can take more than 20 seconds to refresh the FFT if you go too far, you can select the number of points, real/imaginary/modulus, etc.
I guess that, once you have the FFT implemented, it doesn't add any cost in performance terms to give it some flexibility.
(*) I've got an old one and that one was my first exposure to digital oscilloscopes with math operation before purchasing a DS1074Z