Posting a new discussion in a MR diff starts to be slow again when the MR has quite a few discussions

When we have more than ~50 discussions in an MR, posting a new discussions starts to behave slowly again — not as critical as before, but still, perceivable.

We need to investigate ways of reducing the side-effects currently observed when posting a new discussion, as it shouldn't cause this increased delay.

Kapture_2018-09-14_at_13.47.53

This means that time is increasing as discussions grow. By the ~100 discussions, it should be even worse.

/cc @timzallmann @iamphill any ideas?