Its apparent ( from the above text ) that a computers processing power is heavily taxed as one chooses to increase the sweep length .The_REW5_HelpFile said:Length controls the duration of the sweep signal, specifying the number of samples in the sweep sequence. The default is 256k. Dividing the number of samples by the soundcard's sample rate gives the sweep duration in seconds. The overall duration includes silent periods before and after the sweep. Longer sweeps provide higher signal-to-noise ratio (S/N) in the measurements, each doubling of the sweep length improves S/N by almost 3dB. However, the time required to perform the processing after each sweep will more than double for each doubling of sweep length. If REW is running on a computer which does not have a fast processor and a lot of memory, measurements will be much faster using the shortest sweep length (128k samples), at a small S/N penalty of about 3dB compared to the default. 1GB of RAM and a fast processor are recommended if using the 1M sweep, invalid measurements may occur on computers which have insufficient RAM or processor speed
From the looks of your traces, 256k ( the default duration ) would seem to be the sweep length limit for making valid measurements ( on your particular computer ) .
FWIW, I only see 5 traces in your pic, I wonder if the 1M sweep you made was so "invalid" that REW didn't bother to display it ( or maybe you forgot to include it ? ) .