I don't think ten images per second is unreasonable. Of course, it depends on the amount of CPU you can dedicate to it, the complexity of the images, and how efficiently your code is written, but this article talks about GD generating an image in 0.003110 seconds and some digging in the Wayback Machine indicates that those numbers are from over ten years ago; on a modern CPU it should be faster. (The article was a test of the PHP bindings for LibGD, but it's the same underlying C library, so I don't see any reason why the Perl bindings would be significantly slower.)