I can't find the web page anymore, but I do remember that WP devices are supposed to use UHS compliant microSD cards (if a microSD slot is used) to be "certified' by Microsoft as a compliant device to run the WP operating system.
Now the fun part. You can have a microSD card that is Class 6, but is also UHS 1. Meaning that if you put it in a device that is using the old bus technology (aka Class) you will get a transfer rate of a minimum of 6 Mb/s. But put the same card in a device that uses the newer bus technology (aka UHS) you will get a transfer rate of at least 10 Mb/s.
Even more fun. Class and UHS speeds are spec'd as minimum speeds to be certified. So a UHS 1 class card with always get you at least 10 Mb/s. But it could be faster, depending on the manufacturer and how well they make the card, up to 30 Mb/s. Then the card is certified as a UHS 3 card.
Same goes for Class ratings. As soon as the cards transfer rate increases enough to the next level (2 -> 4 -> 6 -> 10).
So...what you should take out of this, is to stop shopping via the "Class" rating and look at the "UHS" rating for microSD cards for Windows Phone.