Spectrogram of 493 signals at a time
"I welcome the addition of a spectrogram showing many channels at one
  particular time interval."


If you don't want to include it, or don't have enough space, it is always worthwhile to put them in a supplementary file, attached to the journal, or separately on the Internet. I suggest the reviewer is looking for an onset phenomenon, and it would be easy to generate a few crops in  time interval $[1500-2500]$ side-by-side, potentially for only a tenth of your data (about 50 signals).