I do not know why but jp2 files are very very heavy on GNU/Linux systems. Opening a folder full of them might very likely stall your file browser or system. Even a small file below 1 M, might take its own sweet time to open in GIMP.Here is a small command to convert a bunch of em into jpg, thought resulting file is usually 1o times larger.
Use this for lossless conversion
mogrify -format jpg -quality 100 *.jp2