Model slowness causes significant UI hang when reordering the scopes list

== Background ==
I recently implemented an autoscrolling mechanism (lp:~josharenson/unity8/autoscroller-alternative) for the ScopesList to resolve lp:1575319 This branch, I believe, is exposing an issue that has likely always been present.

== Steps to Reproduce ==
1. Install the autosceoller branch mentioned above on a mobile device. For the sake of this bug, you can probably just copy the changed qml files.
2. Install enough scopes so that the list is longer than the display; about 20 should work.
3. Open the scopes list, press and hold to enter 'edit' mode, and drag the bottom most scope to the top of the list (which should automatically scroll upwards).
4. Release the scope

Expected behavior: scope drops to new position in list
Actual behavior: UI hangs for several seconds

== Evidence ==
There are several possible culprits for this issue, however I've ruled out two big ones.
Qt - Since the issue isn't present in the mock, its unlikely a Qt bug
Unity8 - I have a qt profiler trace from the unity8-dash that shows no activity at all during the hang. Thus, I believe the issue is somewhere in the scopes or categories model. I've attached the trace for reference.