In my example I have a grid of 96 elements under the parent 'Selectable' element. The parent has a background image with 96 circles. Each inner div contains an image, which in the attached screenshot could have a 'C' on it, or is blue or pink. The highlight class for the Selectable displays grey corners around the circle.

I have observed the following behavior:

1. If I select all 96 objects, then click on the edge of the background to deselect, I have to wait 10 full seconds for that to occur.

2. The more selections I perform, the worse the performance.

3. If I click and drag vertically down, the browser "highlights" the item's images in linear order. In the attached screenshot, I clicked on A9 position and dragged to D9.