SD is slow
I will agree they are very slow. But I use them in old slow hardware anyway so the slowness is not a real problem.
My area of focus is early x86 systems (8086-80386), and I've found that CF/SD/flash -- while having a lower sustained transfer rate than spinning rust on some fast controllers -- are effectively faster than spinning drives because the IOPs are easily 3x or more. Obviously YMMV, but some applications like databases feel like they're running out of a ramdisk on flash storage, vs. being glacial on spinning drives. Same for environments that do a lot of random I/O, like Geoworks.