A longer uploading time is usually related to the speed of your Internet connection. And remember, your maximum download speed often significantly differs from your maximum upload speed.
For example: With a standard internet connection of 16 Mbps, the maximum upload speed is typically around 1 Mbps (128 KB/s). However, this maximum speed is rarely achieved in practice. A more common upload speed would be around 760 Kbps (95 KB/s) on average. For larger files, the upload can take longer.
You can try to avoid longer upload times by resizing your photos before uploading so that you are not uploading the largest size possible, but one that is of sufficient quality to be sold.
You can also choose to upload lower quality images for your customers to choose from, and then swap them out for their higher quality versions via the Manual Revision feature.