This is related to the sampling rate of the analyser and the FFT rate.  It is also important for the realtime spectrum analyzer to give good level accuracy.
Example: for an analyser with  of realtime bandwidth (the maximum RF span that can be processed in realtime) approximately  (complex) are needed.  If the spectrum analyzer produces  an FFT calculation is produced every   For a  FFT a full spectrum is produced  approximately every   This also gives us our overlap rate of 80% (20 μs − 4 μs) / 20 μs = 80%.