CDS has a limited telemetry budget. The normal mode gives CDS approximately 11 Kbits/sec. This corresponds to the capability of transmitting, every second, a single data window of the full 240" slit length, but only about 6 pixels wide!
Since data windows are usually 25-30 pixels wide it is easy to envisage the data pile-up even in a 15-20 second exposure. A full set of NIS1/2 spectra takes 5.3 minutes to telemeter.
Occasionally CDS is allocated double the standard rate, and several studies have been designed to take advantage of that. If you are interested in using that mode, contact the PI to discuss your requirements.