Real-time thumbnails causing large performance issues

When Real-Time Thumbnails are enabled in the content browser and you have many blueprints with static meshes (or anything that is rendered in the blueprint's viewport), the real-time tooltips cause heavy framerate drops when the cursor is hovering over the content browser.

Please note that the effect of this framerate drop is heavier with every extra thumbnail that is added. The blueprints in question also need to be loaded at least once, otherwise the thumbnails are not updated in real-time.

Regression?: Yes
This did not occur in 4.13.2-3172292+++UE4+Release-4.13

Workaround: Disable Real-time Thumbnails in the Content Browser's View Options

1. Download the attached project
2. With the project open, run the following console commands to show framerate and uncap it (you may need a higher number for MaxFPS, depending on your machine):
   stat fps
   t.MaxFPS 600
3. Open the "NewFolder" folder located in the "Content" folder
4. Open each of the blueprints to load them, and then close those windows
5. Click in the viewport and note your current FPS
6. Move your cursor to hover over the content browser (where all the blueprint tooltips are)
   Result: The framerate drops heavily. This effect is more prevalent with more tooltips being rendered
   Expected: The framerate stays the same or at least only drops a reasonable amount