Raw images from digital cameras are way too huge for web sites. Megapixel images would take forever to load. You must optimize your images beforehand in Photoshop or Lightroom for use on the web. Ideally you want to crop and resize the image to the height and width required for the web page. File size should be around 100 KB. I use Photoshop CC 2017, File > Export As > JPG. Adjust the quality slider downward to get the smallest file size possible without diminishing quality too much. See screenshot. I should add that this image was very large because it was used as a page background. Your image height & width values will probably be much smaller than mine.
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How big do you need customers to see the clothing images on your Shopify site? If you try to upload a Nikon D750 image as is, its full image size is 6,016 x 4,016 pixels. If you were able to put that full 24-megapixel image on your Shopify site, anyone downloading it would have enough pixels to make a sharp 16 x 20-inch print from your image. That might not be something you want.
You only need to upload as many pixels as needed for viewing on your web site. For example, if the picture of a shirt only needs to fill the height of an average laptop screen, the image doesn't need to have all 24 megapixels of your camera. I checked macys.com and opened the image of a shirt they sell. The dimensions of that image are 1200 x 1467 pixels tall, which is a little taller than my laptop screen so that buyers can zoom in on details.
Figure out what pixel width and height would work best on your Shopify site, then limit the size of your uploaded copies using the Image Sizing options in the Lightroom export dialog box. You will find that when your images aren't unnecessarily large, they'll take a lot less time to upload too.