I'm not aware of any performance or memory issues with the sample thumbnail filter but what you say could be possible.
If you want to create your own filter, that's possible. This chapter, Flex Filters - 11g Release 1 (22.214.171.124.0), has some instructions. The code for the sample thumbnail filter is also included in the install package, under Sites\FirstSiteII\StoreSchema\src\com\fatwire\firstsite\filter, as well as another sample filter under Sites\FirstSiteII\StoreSchema\src\com\fatwire\firstsite\filter.
I hope this helps.
We have notice some performance issues in our current client so we have modified the default thumbnail filter:
A temporary file is no longer used for the input image. The source image is directly used, as it is not modified by the thumbnail builder.
This avoids some performance issues we detected. First, because of the way the image was being copied (one byte read and written each time), and second, because of some environments' file systems are running poorly.
Also, it was relying on "deleteOnExit", that may produce an impact on the application server performance, as the JVM has to maintain a list of files to be deleted on exit, and that consumes memory. This is even more problematic on servers that are not restarted very often. And if at the end the app server is not shut down gracefully, those files would not be deleted.
An the other hand it seems the thumbnail generator does not need that the source image file ends with the right extension.
Now we directly use the provided source image, it is a ".tmp" file instead of a (e.g.) ".gif" file.
Hope it helps,