Hello Andreas,
you never need a throttle block when you have hardware that
limits/defines your real-world sampling rate; in your case, it's your RF
hardware and your sound card.
Aside from things like implicit resampling in the audio card driver
infrastructure, it's absolutely possible that your bananapi is simply
not performant enough to do GUI, signal processing and audio playback at
once.
The question is: Does the same application work sufficiently well on a
fast PC? Do you see any over- or underrun warnings on your console?
Greetings,
Marcus