As you often can not determine write speed of a SD card by its specs, I use the following linked tool to determine the write speed. It originally was released to test USB-Stick for full capacity, when fake 8 GB sticks were often sold. You need to clear your SD-Card and the tool will then write data onto the whole card an afterwards read the card for errors. At the end it will give you capacity, write and read speed. Only drawback: it is in german. Sorry.
H2testw | heise Download
With this tool I found, as shown in a table in the article linked by Bob, that the Black "Ultra" Sandisk cards are really slow in write speed. Most of my consumer cards of this time where faster in write speed. My reasonable priced Samsung Evo cards outperformed them by far in write and read speed.
If you do not shoot bursts on a regular basis, I think read speed is more important. As you have probably found out, the files of the K-S2 are not so small and if you have to read 16 GB from a SD card with UHS 4 rating you will swap to a faster one. Little side note, as I am shooting jpeg mainly the software to copy the files is even more important. ACDsee for example does index the files while downloading. Result is over 2 hours download for 1000 jpegs from the K-70 and still not through. NikonTransfer: about 30 minutes for all the files.