[2D] Selecting multiple sub-sprites from a spritesheet with many sub-sprites (1000+) causes editor to hang

Steps to Repro:

1. open attached project
2. go to Assets > PixelLevels > Visual > Sprite > Level.png
3. select several of the sub-sprites at once (control-click on them)

Actual: the editor hangs and the user is forced to quit/restart
Expected: the user can successfully select multiple sub-sprites at once

Occurs in 2018.1.2f1 and 2018.2.0b6
Does not occur in 2017.4.4f1, 2017.1.2f1, and 5.6.5p1