Click the Choose Files (aka Browse) button and select two or more local FITS files to load. Choose the number of refreshes and click Run Test. The JS9.RefreshImage() routine will be called repeatedly to update the current display with the image pixels from each file in turn. On completion, the refresh rate is displayed.
The squares.tar.gz file contains two FITS images of size 512x512, 1024x1024, and 2048x2048, respectively, which might be useful for purposes of comparison.