Just saw this, TL;DR we transfer a large file onto and off of the mSD card. If its above 10MB/s for both (which is the base rating on a class 10 card), then it is a fine mSD card. I've also noticed that on multi-gig files speeds tend to drop to around 6MB/s, not the end of the world for us, but also not a sign of a bad mSD either.