As such, Twitter is bringing some of the front-end grunt work back onto the servers in order to speed up load times. As Twitter says, "To connect you to information in real time, it's important for Twitter to be fast." Part of the change is not using # (hashbang) in the permalink URLs to improve initial load times.
Before starting any of this work we added instrumentation to find the performance pain points and identify which categories of users we could serve better. The most important metric we used was "time to first Tweet". This is a measurement we took from a sample of users, (using the Navigation Timing API) of the amount of time it takes from navigation (clicking the link) to viewing the first Tweet on each page's timeline. The metric gives us a good idea of how snappy the site feels.
There are many other improvements discussed in the post on Twitter's engineering blog, so I encourage you to check it out for yourself if you are interested. The Twitter blog post can be found on Twitter's engineering blog here.