The rendering is a bottleneck that's less tolerated than network latency. For example in apps users tolerate a loading spinner more than they tolerate 300ms of

The rendering is a bottleneck that's less tolerated than network latency. For example in apps users tolerate a loading spinner more than they tolerate 300ms of latency when clicking on buttons or when scrolling. It's because of what users have been trained to expect. A loading spinner is considered normal, whereas a small barely-perceptible latency is interpreted as broken behavior when doing actions that in the minds of users should have an immediate response. Of course, for simple websites the rendering time is totally irrelevant. But nowadays we're running apps in our browser. And for example I've seen my browser brought to its knees by pages displaying charts.