It's a real shame that the class system for these cards doesn't say anything definitive about the read performance. I used to like how some vendors would say something like "150x" speed regarding the read performance, but you just don't see that anymore.
Actually, the class system may perversely lead to slower and slower reading performance, because the vendor just slaps a class 6 or class 10 writing performance, barely meeting that metric, and have no incentive to provide faster read performance.
They should update it to have 2 metrics, one for the write and one for the read. E.g., class 6/50 (where 50 is the read performance).