I have a couple of suggestions.
- All pictures a member submits will be of the same dimension. Therefore, once the watermarking is complete for the first picture, we could save that file as a watermark template. Next time, it is enough to copy the image to be watermarked and paste it into this template and just save it as a new image. This way, the watermarking becomes a template process.
- The second option is similar, but a little different. We could complete the watermarking text alone and rasterize it to make it a single layer. Save this small photoshop file containing one layer with the text as the template. When a new image has to be watermarked, just copy this layer from the template on to any image that has to be watermarked.
Both these processes will end up in 5-10 seconds max.