Improve performance of expanding full file in MR diff

Problem to solve

As a follow up of https://gitlab.com/gitlab-org/gitlab-ce/issues/19054, we need to work on improving performance of Expanding Full File in MR Diffs.

After this, we can remove the FF on the expand full file.

Proposal

Priority is that the UI doesn't lock.

What does success look like, and how can we measure that?

Links / references