Post Carousel Block - Flash of featured image seen before posts load

[lizswafford]

Describe the bug
When scrolling down a page that has a Post Carousel Block, the user will see a large featured image appear across the space of the block before seeing the actual carousel with posts. (It is similar to when you see a flash of unstyled text before a custom font loads on a site.)
Tested this on my simple site with different images and posts and found the same effect.

(8657321-zd)

To Reproduce
Steps to reproduce the behavior:

1. Go to a simple site and add a Post Carousel block.
   If the block is not available, activate it in the editor preferences.
2. Set the block to show 4 posts at a time and load 20 or more.
3. Publish the changes and visit the new page.
4. Scroll down to view the block, notice there is a flash of an image before the posts load.
   Reload to view the image flash again, or try a different browser. (This was very noticeable in Safari.)

Screenshots

What the carousel should look like (this screenshot includes custom CSS for the text):

What the carousel looks like when it first loads (the featured image from the first post appears across the block.):

Expected behavior
All posts in the block should load with no pre-loading visible to the user.