Check the documentation for your computer or device if that's why you asked.
U with a one in it is ultra, better than class 10.
SD technology is funny because the larger the card, the slower it is (when filled). With traditional storage devices like hard drives, you have RAM cache providing buffering and the mechanics provide faster seek time.
Defragging the card, if possible helps a lot, but I have a feeling there are going to be a lot of newer faster technologies in this space in the next 15 years.