As we've just said, unless we have suffered it or we are supporting a low-end or really old device, we might not even be aware of what the impact is of not following the performance recommendations or best practices. If we are using a high-end device to test what are we currently developing, we might not see how it performs on a low-end device, and most probably there will be more downloads or more users using it on medium or low-end devices. It is almost the same as when we develop a network-connected piece of software with a nice and reliable Wi-Fi connection, or have an unlimited 4G network. The experience might not be the same for those with a limited or metered connection, and for especially those still on a 2G network.
It is important, in both cases, to take all our target users into account and test under several scenarios, with different devices and hardware.