Google Pixel features an F2.0 aperture lens, which is slower compared to what the competition offers. For example, the Samsung Galaxy S7 and the S7 edge feature a f/1.7 aperture and the LG G5 and iPhone 7 come with a f/1.8 aperture. f/2.0 aperture is not slow by any means, but as more high-end devices come with below-f/2.0 aperture, I've expected Google to do the same.
The aperture, of course, doesn't tell the complete story, and the larger 1.55-micron pixels also helps in improving the low-light performance. A f/1.7 aperture allows around 1.4x more light (1/3 stop) and 1.2x more light than a f/2.0. For a given as same sensor size, it also results in a better shallow depth of field effect (background defocus effect).
The Samsung Galaxy S7 has 1.4 μm pixels, while the iPhone 7 has 1.3-micron pixels. That doesn't prevent both phones from delivering admirable low-light results, and the fast aperture compensates for the smaller pixels for some degree. Because Google Pixel doesn't come with an optical image stabilized lens, it would be useful if the aperture was faster, because you are more limited in using slower shutter speeds to compensate for the lack of light.
So overall I am a bit disappointed that Pixel doesn't have a faster lens, but it's not a deal breaker for me, and we've already seen how good the Google Pixel DxOMark test results turned out to be.