The first time the plugin "runs" it creates the thumbnails on-the-fly and caches them. Depending on your server specs and how many (and how large) images you have, this process may be slow. But it all boils down to the ease of use as regenerating the thumbnails can be as simple as clearing your Joomla's site cache.

If you want thumbnails to remain a longer time in the cache (as they expire after a couple hours), adjust the related interval in the plugin settings (under Advanced). Set the time to a week or month.

This is normal as the related plugin event is fired there too.

What I can do is add an option to only enforce rendering in the article view. In the meantime, edit the file /plugins/content/jw_sig/jw_sig.php and add this after line 59: