Just a side note: slowness during making night shots may be due to noise reduction...
The data transfer speed is easy to calculate. Take a picture of your choice (e.g. RAW with 12-bit, uncompressed - just an example), and see the file size of that. (Or average the sizes of the pictures you have made - or find the worst case size.)
Now, when you make pictures, you first fill up the internal buffer, and meanwhile start writing the files on the SD card. So you will have to know how many pictures you can make in a burts, before your camera slows down considerably. This you can test with any card, the slower the better, actually.
So let's say you have 10 MByte pictures, and your burst is 9 picture per second. (I am completely making these numbers up.) So your data transfer rate should be 10*9 = 90 MByte/sec. And you select a card that supports that speed, easy. (You should choose a little faster card than what your calculations shows, just to leave some time for filesystem manipulation, access times, etc.)
Now, beware. There are MANY counterfeit, fake, etc. memory cards which do not support the writing speed they advertise. E.g. Amazon is known to NOT be a reliable Sandisk/Kingston source... So test your card for writing speed when it arrives.
Now, if you want video, check your manual for data transfer rate specification, they usually say that you need 45 MB/sec or so, and you just need a card that supports it...
