I got the Samsung UHS-1 from Amazon as well, tested it and I am very happy with this investment of $65. Now when I do a TWRP backup, it finishes in 212 seconds for 1850 MB, averaging 8.7MB per second. When I copy files over from a PC, using the Silicon Power USB 3.0 card reader which is UHS-1 compatible, I constant have a write speed of 14.5 MB/s and the Read speed is over 27 MB/s.
They advertise to have a read speed of 70 and write speed of 20 though.
Put your SD in your card reader and plug it in window PC. You need to reformat your fat32 partition again in window. In the window format menu, don't use the default format setting, instead using the 64k cluster or slot in the pull down menu. Now day, most of files are huge and no need to set your cluster so small. It should increase your read write speeds. Good luck..