The speed and/or type of your internet connection combined with the file size of your images are the main factors that affect how long it takes to upload. 

Check to see how large your images are. Reducing their size can speed up the upload process. 

They do not need to be larger than 1200 pixels on the long side.

It's best to upload images that are 5MB or less. The maximum file size for image uploads is 20MB. The larger your image file, the longer the upload process will take.

Please refer to this FAQ: Image Upload Size.