I'm running G3 on a VIA 1GHz board with 1G RAM so it is already quite challenged considering what it is doing. It takes about 30-40 seconds for an image. Don't forget that it isn't just uploading the image, it is also generating the low res version and thumbnail. Then any other module you have installed may be performing any of its operations as well.
For comparison, I read that your Zyxel is a 1.6GHz SoC board. It is probably challenged as well - and its probably doing all of this on top of its normal NSA functions.