I want to give a big +1 to support this feature. I just installed Affinity Photo trial and it is a wonderful product. But a big part of being a web developer is finding the right balance between quality and file size, and like previous posters said, every photo is different.
Some photos are used as full page backgrounds and don't have fine detail, so quality can and SHOULD be notched down as much as possible (for large background images). Others do have fine detail and may need to be bumped up high on quality. Some photos I can get down to 30% quality, while others I need 80%. There is quite a range here. You can do this very quickly in Photoshop by nudging the quality bar until you find something just right. This will be a lot slower in Affinity by having to save and preview the save, go back etc.
Moreover, from an SEO standpoint, Google cares about how long it takes to load a website, especially for mobile. The more kilobytes you can save with optimized images, the better.