The iPhone 8 comes with an f/1.8 aperture lens camera, which is the same one as the iPhone 7. We've already seen more and more high-end and even mid-range devices coming with lens apertures lower than f/1.8. For example, we have the LG V30 rear-facing camera that features an f/1.6 aperture lens, the Galaxy J7+ comes with F1.7 lens, the Galaxy S8 comes with F1.7 lens for both the rear and front cameras, etc.
The main trend is to bring users cameras that have better low-light capabilities and one way to do this is to introduce a wider aperture lens. For me it seems that Apple certainly rushed this product out, even was rushing to announced it so it will have more time talking about the new iPhone X. It's like Apple didn't care about making the iPhone 8 camera faster.
The good news is that there is an Optical Image Stabilization (OIS) and we can still expect this sensor to perform really well, even under low-light conditions. The thing is that it could have performed better had Apple opted for a faster lens. It doesn't necessarily means that the phone with the faster aperture will result in a better low-light performance, there are many other factors like pixel size, sensor type, ISO range, optical performance and other factors, which influence the image quality and low light capabilities. However, as a new 2017 iPhone, I would expected more from Apple.