Minor performance issue on shift + down, shift + up on includes

[Scott Kostyshak]

To reproduce:

1. Move the attached file in the lib/docs directory.
2. Open the file.
3. Put the cursor before, e.g., the Additional.lyx include.
4. Press Shift + Down (or Shift + Up).

Result is that there is a small pause before the selection is finished.
Subsequent presses extending the selection in the same direction are
almost instantaneous, so it's just the first one.

In case it's hard to notice, you can try the following, after (3) hold
shift and then press Down. After the selection gets to the bottom, keep
holding shift but change from holding Down to holding Up. You'll notice
that the cursor gets stuck for a small amount of time at the anchor
point.

Strangely, I can reproduce if I select with Shift + Right, which also
selects all of the insets. This surprised me, and maybe it provides a
clue.

Can anyone reproduce?

Scott
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