Is the class rating of my MicroSD card relevant?
If you've upgraded the SD card on your phone to a higher class, have you measured or noticed any significant performance difference? What kinds of apps seemed most affected?
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
A higher class card is going to give you faster read and write speeds to your SD card. So any apps that use or write data to the memory card will get a boost (one example would be the camera and gallery apps). Another possible example would be apps saved to the memory card. You probably, however, won't be able to tell a huge difference, or at least it won't be as noticeable as it would on a DSLR camera where high speed cards make a world of difference when taking fast, continuous shots (sports).