Ok, that makes sense now. I still don't understand why a NPOT texture would take longer to upload to the card. Are you sure your driver supports NPOT textures? I just tried it here and a 1023x1023 texture took just as long as a 1024x1024 texture. When you resized your texture was it resizing down to the nearest power of two? That would explain why it would take less time, since there would be less data to upload.
Either way, we will consider adding manual scaling support to texture objects in a future release. In the meantime you can use the PIL library to scale images. I've actually used this library to do exactly what you are talking about, load and scale images in a background thread, then upload to the card in the main thread. This
article describes how to install and use the PIL library with Vizard.