How to group thousand of data points for each user
The solution was to use run-length encoding.
Let's say that I have:

The first 10 questions true
The next 24 false
The next 34 unknown

I'd transform it in 10T24F34U.
You can find more info on Wikipedia's Run-length encoding as provided by D.W. in the comments.
The main reason for this solution is to send data via an API. I'm not sure if I'm going to use this algorithm or just rely on the typical compression when sending data to a browser. So that is another possible solution.