I have worked for a web design company. A project manager always ask me to make sharp images on the web site, it means the JPG file need 100% quality and not optimized, she told me she was a graphic designer.
The size of 100% quality JPG is double of 80% quality!
80% quality is good enough for web site, especially for e-commerce web site. The speed could be a big problem when the size of images are double.