Hello
use some common logic here ...
it uses phpThumb to create thumbnails,
1. it will make very big difference the size of the original images,
- the larger they are
the more time you need to create thumbnails
e.g. imagine original images being 4 MBs compared to being 200 KBs
2. also the cache maybe too small and thumbnails are not cached for long
(see our performance guide FAQ article for changing phpThumb cache size )