SD Cards
The package for a SD card may say Ultra High Speed Pro, but the markings on the card tell the truth. These markings are defined by the SD Card association. The write speed is slower than the read speed, since the memory must be cleared before writing new data.
Capacity markings
SD up to 2 Gbytes
SDHC (High capacity) 4G to 32G
SDXC (Extended capacity) 64G to 2Terabytes
Standard SD Cards
The maximum standard SD interface speed is 25 MBytes/second.
Write speed markings
circle 2 - 2 Mbytes/sec
circle 4 - 4 Mbytes/sec
circle 6 - 6 Mbytes/sec
circle 8 - 8 Mbytes/sec
circle 10 - 10 Mbytes/sec
Interface speeds
(1) Default Speed mode: 3.3V signaling, Frequency up to 25 MHz, up to 12.5 MB/sec
(2) High Speed mode: 3.3V signaling, Frequency up to 50 MHz, up to 25 MB/sec
UHS SD Cards
The UHS-I and UHS_II specifications adds faster interface speeds. Note that the host controller must support these interface options. The UHS-I interface runs at up to 104 Mbytes/sec, and the UHS-II up to 312Mbyte/sec.
Write speed markings
U1 - 10 Mbytes/sec write speed
U3 - 30 Mbytes/sec write speed
Interface speeds supported
(1) Default Speed mode: 3.3V signaling, Frequency up to 25 MHz, up to 12.5 MB/sec
(2) High Speed mode: 3.3V signaling, Frequency up to 50 MHz, up to 25 MB/sec
(3) SDR12: UHS-I 1.8V signaling, Frequency up to 25 MHz, up to 12.5MB/sec
(4) SDR25: UHS-I 1.8V signaling, Frequency up to 50 MHz, up to 25MB/sec
(5) SDR50: UHS-I 1.8V signaling, Frequency up to 100 MHz, up to 50MB/sec
(6) SDR104: UHS-I 1.8V signaling, Frequency up to 208 MHz, up to 104MB/sec
(7) DDR50: UHS-I 1.8V signaling, Frequency up to 50 MHz, sampled on both clock edges, up to 50MB/sec
(8) UHS156: UHS-II RCLK Frequency Range 26MHz - 52MHz, up to 1.56Gbps per lane.
(9) UHS312: UHS-II RCLK Frequency Range 26MHz - 52MHz, up to 1.56Gbps two lanes half duplex
The UHS class 3 devices are available from a few vendors for about the same price, why would you buy anything else?
Link to the SD cards tested with a Raspberry PI. [1]