@Loki, I managed to find my post (I’m tired now to explain everything on how I’ve tested) yet creating test_eMMC_zero_data file of preferable size is important. Please follow this procedure that I made when I had more time, hope that helps:
Afterwards, when ready to use above created file (within that folder) you might like to take a look (proceed) with something as posted inside of my post #40, same thread. All actually based on @amosbatto request, his post #24.
I’d also like to see your Librem 5 eMMC performance results (therefore this post), sure, if you don’t mind.