The advertised speeds don't mean much (to us).
SD cards are optimized for sequential r/w, because that's what is needed for photo bursts and video recording.
What we want is random r/w, because that's what matters when accessing a lot of small files, like libraries and configs of a linux system. SD cards are much worse on this matter than SSDs, even if their sequential speeds look somewhat similar.
This site [1] has a nice amount of benchmarks of different SD cards, including random r/w. According to these tests Sandisk Extreme UHS I U3 SDXC cards seem to be the fastest for our use-case, both for full-size SD and µSD. Other Sandisk Extreme and Samsung's Pro Plus µSD cards come close.
What you can see here is that even the best cards don't reach 4MB/s in random r/w, while real SSDs are between 20 and 100MB/s afaik (my knowledge here might be outdated).
You can also see that having a UHS II card doesn't seem to help at all unless you have a UHS II card reader (which the Pyra doesn't).
[1]
http://www.cameramemoryspeed.com/reviews/sd-cards/