When you add images via the Bulk Upload, Asset Bank will generate thumbnails for each one as it goes along. If the images are very high-res, this thumbnail generation could take a while.
It is possible to configure Asset Bank so that it defers the generation of thumbnails. This means that it will import the files in your bulk upload much more quickly - however the thumbnails will be created in the background and therefore may not appear (in lists of assets etc) until later.
If you want to give users performing bulk uploads the option of 'deferring thumbnail creation', i.e. doing it in the background, then set the following setting to true in the ApplicationSettings.properties file :
Now you will see a new 'defer thumbnail creation tickbox when uploading. Ticking this will allow you to directly proceed with adding metadata via Batch Update (i.e. you will not have to wait for all Thumbnails to be generated first).