Windows: Shift+up/down arrow selection feels sluggish

[peterflynn]

Steps:

1. Open a largish file like DocumentManager
2. Place the cursor at the start of some line
3. Hold down Shift + down arrow to select a range of lines

Result:
Feels pretty sluggish. At the start, there is often a noticeable pause after which the selection will jump to about 5-10 lines larger... then the selection grows more smoothly, but still feels quite sluggish compared to Mac.

Expected:
Should be as smooth as on Mac.

I also tested this in CodeMirror on Chrome on the same Windows machine, and it felt better there too.

[peterflynn]

Note: dragging to make a large selection feels pretty slow too, but it also feels slow on my Mac. There is some difference there too though: on Windows it seems to pause frequently so that the selection jumps in big increments (similar to the start of a shift+arrow selection... though it feels a lot worse because dragging moves so much faster); on Mac it repaints frequently but consistently lags the mouse position by a few lines.

[jlondon]

This and #362 should be part of a research spike.

[njx]

Removing Sprint 5 tag

[jlondon]

Moved to backlog - closing.