This experiment compares the speed of microSD cards: Sandisk Extreme 256GB vs. Sandisk Extreme 32GB. Sandisk Ultra 32GB is included as baseline. All cards are rated as Class 10. According to Sandisk website, Extreme-branded microSD cards have up to 160MB/s read speed and up to 90 MB/s write speed. Both Sandisk Extreme cards are UHS-I U3 enabled, but only the 256GB model is rated with V30 and A2 symbols. Theoretically, A2 speed rating means that a memory card should perform better to run applications in mobile device because it is capable to perform 4000 IOPS (input-output access per second) random read and 2000 IOPS random write. V30 rating means the card can sustain at least 30MB/s continuous video recording. Let’s see if those new ratings are actually reflected in simple file copy operations.

For this test, I use a Mac mini with built-in USB 3.0 ports. The memory cards are accessed through a USB 3.0 memory card reader (Transcend RDF5). I prepared three sets of data to be written to, and read from the micro SD cards:

  • Data A is 1 large file. The size is 4.25 GB.
  • Data B is a set of 12 medium files. The total size is 5.81 GB.
  • Data C is a set of 45,976 small files. The total size is 5.47 GB.

Testing process is simple. First I formatted each memory card using ExFAT so at the beginning of the test, they are empty. The formatting uses standard ExFAT from Windows with all default settings. To measure WRITE speed, I copied each set of data into each memory card and timed the process. To measure READ speed, I copied back those sets of data back into my computer and timed the process. To perform the file transfers, I use a tool named ChronoSync 4.9.10. This is a backup app for Mac, but I have good experience using it as file copier because it performs generally faster than using the standard Finder.

OperationDataSandisk Ultra 32Sandisk Extreme 32Sandisk Extreme 256
WRITEA5 mins 15 secs
avg speed 13.82 MB/s
1 mins 29 secs
avg speed 48.90 MB/s
58 secs
avg speed 75.03 MB/s
WRITEB7 mins 22 secs
avg speed 13.46 MB/s
1 min 55 secs
avg speed 51.73 MB/s
1 min 9 secs
avg speed 86.22 MB/s
WRITEC58 mins 32 secs
avg speed 1.59 MB/s
16 mins 13 secs
avg speed 5.76 MB/s
18 mins 9 secs
avg speed 5.14 MB/s
READA1 min 4 secs
avg speed 68.00 MB/s
1 min 2 secs
avg speed 70.19 MB/s
49 secs
avg speed 88.82 MB/s
READB2 mins 20 secs
avg speed 42.49 MB/s
1 min 30 secs
avg speed 66.10 MB/s
1 min 7 secs
avg speed 88.79 MB/s
READC13 mins 17 secs
avg speed 7.03 MB/s
2 mins 45 secs
avg speed 33.95 MB/s
3 mins 30 secs
avg speed 26.67 MB/s

As expected, Sandisk Extreme 256 GB with V30 and A2 ratings is fast. Curiously, it does not win all speed tests. For data C (massive amount of small files), Sandisk Extreme 32GB actually performs better. Perhaps this is related to the significantly larger storage capacity (32GB vs. 256GB).

Back in 2015, I performed similar speed comparison using the same Mac mini computer with the same Sandisk Ultra 32GB and Sandisk Extreme 32 GB microSD cards. You can find the speed test results here. The experiment from 5 years ago used exactly the same sets of data files (A, B and C). The differences between these 2 experiments are: I have upgraded my Mac mini storage to SSD (the data from 2015 used Mac mini with HDD), use newer MacOS (currently 10.14.6) and ChronoSync 4.9.10 today. If you compare the numbers, you will notice that the memory cards generally perform better with my Mac using SSD. One exception is the “write” operation of Data C for Sandisk Ultra 32 GB, it performs worse. I repeated this particular test 3 times and ended up with similar result, I could not figure out the reason.