I don't often deal with jpeg2000 files but I recently had to work with some old book scans (appears not uncommon for librarians to rely on this formats when scanning their collection).
and much to my surprise, it appeared that xnview/xnviewMP took a very very very long time to render the thumbnails.
so I did some test, and here are the results:
one folder with 500 .jp2 files (each file 3065x4676 - 24 bit per pixel - average file size 390KB)
- Xnview: 50 min to render the 500 thumbnails.
- acdsee: 5 sec to render the 500 thumbnails.
at first, given the huge difference I though acdsee was probably only reading some embedded thumbnail or something, but no, it does reads and display the fullsize in a snap.
may be worth finding out what jp2 lib is acdsee using... or perhaps consider finding a more efficient decoder for xnview.
(btw I had to copy the jpeg 2000 plugin from xnconvert to be able to open the files in xnviewMp. before that it did display the thumbnail but would not open the file (right click > open was grayed out)
Anyway, it looks like xnview won't be up to the task as I have about 9000 jp2 files to go through