It is usually the loading of many resources (js etc) that takes time. If you just send some html (and css) to the browser it can show it quickly. But companies

It is usually the loading of many resources (js etc) that takes time. If you just send some html (and css) to the browser it can show it quickly. But companies like Facebook and Airbnb are starting to send html first and then use javascript for everything. This makes the first pageview very fast while still keeping the flexibility and other good parts of a SPA system.