Pictures blurry, pixelated in Vintage Prints theme slideshow

I have spent a great part of the past 6 months trying to find something, anything on an issue I have been suffering with, but I haven't found much if not anything! Apparently, I must be the only one who suffered from this::

I have a MacBook Pro 13" Early 2015, and ever since I upgraded to macOS Mojave, I've had a problem with the Photos app and viewing my photos in a slideshow, particularly using the Vintage Prints theme (which is my favorite theme), or pretty much any other theme besides Classic. The problem is, after a few pictures pass, they start looking pixelated or blurry; and this is happening with the main picture, as well as the background ones.

I never had this problem with El Capitan, Sierra, or High Sierra; it only started with Mojave. Today, I finally rolled up my sleeves and started to tackle this problem. In the past 6 hours, I have re-installed macOS Mojave, i tested the slideshow first thing after logging in, the problem remained. Then I did a Clean Install of macOS Mojave (disk utility>delete>install macOS), and yes, the problem was still there. So just now, I reverted back to macOS High Sierra, and the problem has gone away! (Yay!!!!!!!!)

-however, now I have a new problem, the Photos Library I backed up from Mojave will not load into Photos on High Sierra, and that's 89 GB worth of photos (Noooooo!!!!!!!!)

Anyhow, that's for a new topic of discussion. I wanted to make sure I shared my findings, so the next person out there who loves the Vintage Prints slideshow theme (as much as I do), and upgraded to Mojave, and is suffering this terrible bug/problem/issue like I was, will know they're not alone! (and that they need to revert back to High Sierra to fix the problem, but beware of the Photos Library incompatibility -you might want to consider enabling iCloud Library).