The Cards

We decided to follow-up on our test of the SanDisk Ultra SD and microSD cards with a comparison of flash cards from various generations. We conducted all tests using a Kingston FCR-HS3 USB 3.0 Media Reader, connected to a USB 3.0 port on a Asus Maximus V Gene Z77 motherboard. To determine just how much flash card type and rating mattered, we tested a very wide range of cards, as follows:

microSD Cards

  1. Kingston 1GB microSD (unrated - likely Class 2)
  2. Kingston 4GB microSDHC Class 4
  3. Transcend 8GB microSDHC Class 6
  4. Transcend 32GB microSDHC Class 4
  5. Sandisk Ultra 32GB microSDHC UHS1
  6. Sandisk Ultra 64GB microSDXC UHS1

SD Cards

  1. Transcend 4GB SD 150x
  2. Sandisk Ultra 64GB SDXC Class 10

CompactFlash Cards

  1. Transcend 4GB CompactFlash 120x
  2. Transcend 8GB CompactFlash 266x
  3. Transcend 16GB CompactFlash 400x

As you'll see, despite the wide range of speed ratings, these products actually line up fairly neatly into speed categories. But of course the details matter, so have a look at our results:


Essentially, despite the variety of ratings, there are four speed classes evident: legacy (~60x), mid-range (~133x), high-speed (~266x speed) and ultra high-speed (400x). We haven't tested any of the newer 600x and higher CompactFlash cards, but we presume they'll perform better than our 400x model. Keep in mind that the old "x" speed rating used as its baseline the sequential read speed of the original CD-ROM, which read at 150 KB/s. So 400x should theoretically have a sequential read speed of 60,000 KB/s, or 60 MB/s. Since this was relatively arbitrary, speed ratings by "class" were developed. This rating system is in turn now being set aside for a pure MB/s rating, such as on SanDisk's latest products.

The real shocker here is that regardless of rating, the new high-end SD cards simply do not compare to even midrange Compact Flash cards in regards to write performance, which is what really matters for use in cameras. That is likely why professional-grade cameras still use CompactFlash, despite its larger size. The 400x-rated CompactFlash card in this test simply runs away from the competition, with read speeds nearly 100% faster than the next fastest card, and write speeds significantly faster as well. It's possible that the SD and microSD cards are being limited by SDIO, their card interface, and may be reaching a plateau available in that card format. SanDisk now markets even higher-speed products than those in its Ultra line, and we wonder if these Extreme and Extreme Pro cards would in fact catch up to older CompactFlash cards.

For application performance, such as on a smartphone, the newest SD and microSD cards all perform similarly, nearly matching the CompactFlash cards in 512K read and 4K read performance, which the CompactFlash cards were likely never designed to excel at.

Overall, it's eye-opening to see the progress that has been made in SD card speeds since the introduction of the standard. We've essentially found a five-fold increase in read speeds between Class 2 and Class 10 products, but these still pale in comparison to CompactFlash. One issue of note - our five fastest cards demonstrate read performance that would clearly be bottlenecked on a USB 2.0 reader, so if you frequently use flash cards for large file transfers, make sure to invest in a USB 3.0 reader.