Lag showing images in Windows

I made a simple app that crossfades random images from a folder. The thing runs smoothly in my Mac (old 2014 with High Sierra, 8GB RAM) but when I run it in my Windows box (high end lots of CPU and RAM with a big GeForce RTX 2070 and SSD drive) the images appear lagging, the crossfades aren’t smooth, like the app is pausing every few seconds loading the next image (I assume). Any ideas why this could happen? Is the exact same code.

Thanks for any help, this is driving me nuts!

How many images? are allocating them at the beginning or while the app is running?

It could be thousands of images in max res 1800x1800 px. For this reason it should be dynamic load (but I’m considering batch memory loading of a subset if necessary)
The idea is just a slideshow that mixes 4 layers of images continuously. My SSD disk marks 250MB/s so it can’t be in the loading (my disk in the Mac box is an old HDD and it runs smoothly)…