When shooting digital photographs, you can usually feel the difference between a cheap Class 4 SD card, with its minimum 4 MB/s write speeds, and a faster Class 10 or UHS (Ultra High Speed) card. SD speed classes can be really confusing--Class 10 cards are technically the fastest, with required 10 MB/s minimum read and write speeds, but there's a world of difference between a basic Class 10 card and a Class 10 UHS card, which can operate at quadruple the standard SD clock speed. Slap one of those cards, like the SanDisk Extreme Pro, into a camera, and you'll feel the difference--photos write to the card in a snap and reviewing a shot won't leave you staring at a blinking LED for three seconds.
The speed of the card makes a difference, but a fast card isn't guaranteed to reach its potential in every device you use it in. Devices like digital cameras talk to SD cards with host controllers, and those host controllers can vary in speed and compatibility. For example, older host controllers only support the SD and SDHC formats, not the more recent SDXC. Using a really fast SD card with a slow host controller is a bit like plugging a USB 3.0 flash drive into a USB 1.0 port. You're not going to come close to maxing out what the card is capable of.
Unfortunately, age isn't the only factor that accounts for SD controller performance. Last year, while researching SD cards for The Wirecutter, I talked to Nikon, SanDisk, and some photographers, including the experts at Imaging Resource. I also looked at Rob Galbraith's extensive database of SD and CompactFlash performance numbers. Next time you buy an SD card--or anything that uses one--keep in mind that hardware like the memory controller and CPU, even in a brand new camera, may dramatically undercut what the card should be able to deliver.
A Slow SD Card Controller in a Camera: Canon 5D Mark III
Here's an example. Galbraith tested out the Canon 5D Mark III with a variety of CompactFlash and SD cards. The fastest CompactFlash cards could write JPEGs at around 46 MB/s, and RAW files around 80 MB/s. However, the fastest-performing SD cards could only write at 18 or 19 MB/s. There were two SD cards that delivered nearly identical performance: one was a Class 6, and one was a CLass 10 UHS-I SanDisk Extreme Pro--which should be capable of write speeds up to 95 MB/s.
Yikes. At first, it seems bad that a Class 6 card would be faster than a card claiming to deliver 95 MB/s speeds. But quite a few of the SD cards Galbraith tested in the 5D Mark III actually delivered very similar speeds. Basically, they were hitting a performance wall.
When I emailed with Imaging Resource, they wrote something similar: "Memory card performance varies a lot by camera, so we can't really draw any general conclusions." Here's an example where that unpredictability actually justifies that 95 MB/s card: "Our first 95MB/s SanDisk Extreme Pro card...was up to ~3x faster at buffer clearing than the 45MB/s card with the Olympus E-M5. Our E-M5 results for example aren't what you'd expect for a card that is only rated twice as fast (it's rated for 90MB/s writes), and we've seen cards with slower ratings perform better than cards with faster ratings on some cameras."
This unpredictability can make buying new hardware a real pain. The Canon 5D Mark III is a $3500 camera. For that price, wouldn't you expect a fast SD card slot? Canon's spec sheet only lists "SD, SDHC, and SDXC Memory Cards," a broad description that could easily include Ultra High Speed cards, which are backwards compatible, and thus work just fine in any device that supports SDHC.
This is a device that should read SD cards much more quickly--it's relatively new gear, and costs a lot of money--but doesn't.
And in the 5D Mark III's case, that slow memory controller is actually a serious liability. Writes photographer Jeff Cable: "YOU DO NOT want to put a card in the SD slot. Why? Because, for some reason unbeknownst to me, Canon decided to build the 5D Mark III with one very fast CF slot which supports the newer UDMA7 protocol and a standard SD card slot which does NOT support the high speed standard (called UHS – for Ultra High Speed). This is really strange because many other cameras have come out with UHS1 compatible slots over the last year. Without UHS support, the top speed that can be achieved by the SD card is 133x...The 5D Mark III defaults to the slowest card that is in the camera at the time. If you want to take full advantage of your professional CF card, leave the SD slot alone–save it for times when speed isn’t important but having backups or more storage is."
Is this slow SD card slot a reason not to buy the 5D Mark III? Not necessarily--most photographers will be happy using the much faster CompactFlash slot. But this is a device that should read SD cards much more quickly--it's relatively new gear, and costs a lot of money--but doesn't.
Lesson: If you're buying a camera, especially one that's only outfitted with an SD card slot, make sure the controller isn't going to be a disappointing bottleneck.
A Slow SD Card Controller in a Laptop: 2011 MacBook Pro
If a laptop has an SD card slot, you'll likely see that fact mentioned in a review. It's a convenient feature to have. What you probably won't see, though, is a test of how speedy that SD card slot is. Granted, speed in a laptop's card slot is far less important than the speed in a camera, where you want to be able to clear the buffer as quickly as possible and potentially write many photos in rapid succession. Worst case, in a laptop: Your photos take longer to transfer.
Here's a great segment from Anandtech's review of the MacBook Pro with Retina released in summer 2012, which goes the extra mile to test the laptop's SD card reader:
"It was our own Brian Klug who clued me into the horrible behavior of the 2011 MacBook Pro’s SD card reader. Depending on the SD card used, the integrated SD card reader either performed admirably or was the most frustrating part of the Mac experience. Out of the three SD cards I frequently use: a Patriot LX series card, a Transcend and a new UHS-I Patriot EP Pro, only the Transcend card actually works remotely well with the 2011 chassis. Even then, it’s not perfect. I usually have to insert and remove the card at least once before the reader will recognize it. The LX and EP Pro on the other hand are measurably worse. To get the EP Pro to work in the 2011 MBP’s reader I usually have to push the card in then apply upward or downward force to the exposed edge of the card to get it to read properly. Even then it’ll usually disappear from OS X or be present but read at bytes per second. I doubt this is the fault of the card itself but rather the latest example of incompatibility with the horrible SD card reader in last year’s MacBook Pro.
"At least with the cards I’ve tested, the Retina MacBook Pro exhibits none of these issues. Over dozens of insertions I had no issues reading from or writing to all three of these cards, including the problematic ones. I ran a Quick Bench test on the EP Pro as it’s the fastest of the lot and came away with reasonable performance as well. Roughly 80MB/s reads and 40MB/s writes. The numbers are shy of Patriot’s 90/50 spec but quite good."
For comparison, the 2011 MacBook Pro could read at 80 MB/s from the same card, but could only write at about 10 MB/s. That's a bad SD card slot. Again, read speeds are more important here, in general, but some people now use SD cards as expanded storage in laptops. Systems with SSDs often only come with 128GB or 256GB of storage, so an extra 32GB or 64GB can be substantial. Slow write speeds would destroy that use case.
Slow SD Card Controllers in External Readers: Too Many to Name
Back to Rob Galbraith's thorough testing for this one. In addition to testing camera read and write speeds, Galbraith tested the read speeds of 19 memory card readers. The tests are from 2011, so card readers released in the last two years are probably, in general, faster.
But plenty of the readers Galbraith tested showed big variances in performance. A Delkin Elite 633 card scored read speeds of between 69 MB/s and 83 MB/s in a Lexar USB 3.0 reader, a Pretec 240 reader, and a SanDisk Extreme Pro Reader. But in other readers from Addonics, Griffin, Hoodman, and Lexar, this time in a USB 2.0 flavor? Scores of about 17 MB/s or 20 MB/s.
The SanDisk Extreme Pro's results look similar. Unsurprisingly, the best performing SD readers are USB 3.0, rather than 2.0, devices. USB 2.0 should be able to deliver speeds up to 35 MB/s, but for whatever reason--the quality of the memory controllers in these readers, or overhead from USB drivers--they regularly top out at 20 MB/s. That's something to keep in mind if you plan on buying a new card reader. With USB 3.0, you should be able to come close to maxing out the speed of a high-end UHS-I SD card.