There's an excellent benchmarking post on SD card performance on Raspberry Pi's by Jeff Geerling. These are the results of tests on my SD cards.
I've created a script that runs Jeff's test here.
hdparm is a sequential read test, nice for quick boot
dd write is a block read/write test
Type,Device,hdparm,dd write,4k rand read,4k rand write
USB SSD,WD PiDrive 314,17.93 MB/s,36.3 MB/s,0.466 MB/s,1.5 MB/s
Micro SDHC,16GB Samsung Pro,tbd,tbd,tbd,tbd
USB,128GB SanDisk USB,33.25 MB/s,21.7 MB/s,2.53 MB/s,1.53 MB/s
USB,64GB Trascend USB,35.56 MB/s,15.2 MB/s,4.08 MB/s,0.62 MB/s
Micro SDHC,32GB Samsung EVO,20.17 MB/s,15.0 MB/s,8.61 MB/s,3.05 MB/s
Micro SDHC,8GB SanDisk,21.64 MB/s,9.8 MB/s,7.51 MB/s,0.76 MB/s
Micro SDHC,32GB GSkill,20.6 MB/s,16.3 MB/s,4.0 MB/s,0.6 MB/s
Micro SDHC,16GB Samsung EVO,9.82 MB/s,11.7 MB/s,,
Micro SDHC,32GB SP,17.83 MB/s,10.3 MB/s,2.5 MB/s,0.25 MB/s