What should I do if it is going to take a while to gather and process data for template?

I am a budding web dev, so I have yet to learn a lot of best practices. I'm very new to Flask and wondering what I should do if it is going to take a while to gather data to show to the user.

Say I am querying reddit and displaying a list of posts made by the logged in user. The process of gathering posts (and doing the secret processing) may take several seconds. How can I display a page to the user and populate it slowly or perhaps redirect when the final page is ready? What is normally done in a situation like this? Some kind of ajax magic?