Why are your SD cards slow?

Why read this article? This article has been raved about by readers:

Shawn (this is) the best article I have ever seen on SD’s and devices transfer speed. I have been building computers since before the mouse was invented and (this) article sums it up. And about 80 percent of people don’t even realize this.
Thank you.

This article provides all the information about SD cards and USB card readers that you could possibly need. The reasons why things could be messed up with your data transfers are listed below. Please read each section carefully in order to make sure you understand exactly what is going on.

#1 Your SD card is actually slow

SD cards have been around awhile and because of that there is a chance you dug up an old card somewhere that is older and slower than any of your other cards. In this case there’s not a lot you can do, and since the SD card standard is really old, if you’ve been doing media stuff for awhile there’s every change in the world you have a slower card.

The original SD cards don’t have any extra naming on them, it’s just “SD”. SD cards have been introduced according to size class, below are the different size classes currently offered:

Maximum Capacity	2GB	32GB	2TB	128TB
Date Introduced	1999	2006	2009	2018

Size classes are a broad category for different SD card technologies that show what the maximum possible size is supported by each technology. Since 2018 most new cameras should support the largest size class for SD cards which is SDUC. These labels should be printed somewhere on your SD card, so, if you see SDHC which was introduced in 2006, you may have an older and slower SD card.

The most important rating related to speed is the Bus Interface type. Look on your card to see if it has a UHS Bus Logo on it as shown in the chart below. If there is no logo your card is probably an older and slower technology. If your card one of the bus logos in the chart below, then you should get the associated Bus Speed. However, that doesn’t mean that you’ll get those speeds for bursts or sustained writes, although you should something in that range for bursts because there is obviously no point in supporting a faster bus speed if the memory in the card doesn’t support the bus speed.

Bus Interface	Bus Logo	Bus Speed	SD	SDHC	SDXC	SDUC
Default Speed		12.5 MB/s	yes	yes	yes	yes
High Speed		25 MB/s	yes	yes	yes	yes
UHS-I		50-104 MB/s	no	yes	yes	yes
UHS-II		156-312 MB/s	no	yes	yes	yes
UHS-III		312-624 MB/s	no	yes	yes	yes
SD EXPRESS		985-3983 MB/s	no	yes	yes	yes

Bus speed does not guarantee sustained writes. It guarantees that the card can write in a burst at up to that speed. For sustained writes you’ll need to refer to the following chart which is called the VIDEO SPEED CLASS.

Minimum Sequential Write	Max Bitrate (Video Quality)	Video Speed Class	Speed Class Logo
6 MB/s	45 Mbit/s	Class 6
10 MB/s	75 Mbit/s	Class 10
30 MB/s	220 Mbit/s	Class 30
60 MB/s	460 Mbit/s	Class 60
90 MB/s	700 Mbit/s	Class 90

With the Video Speed Class, it’s purely about sustained writes specifically for something like recording video. Sustained writes tend to be a lot lower because these writes are limited by the flash memory in the SD card as opposed to the card’s bus.

#2 you’re using an old USB port or an old USB hub

Assuming you have determined that you have a relatively fast SD card to work with we can now move on to the next possible issue, your USB port or hub. From 1996 to 2014 there were 4 speed classes of USB introduced and they all used the same familiar rectangular USB plug known as a USB A connector. USB 3.2 introduced in 2017 uses only the new USB C type port. However, just having USB C does NOT guarantee USB 3.2 speeds! USB C is backwards compatible all the way to USB 2.0 so a USB C device could support any of the USB specifications from 2.0 all the way to the upcoming USB4… To try to state that as clearly as possible, USB 3.2 and USB4 ONLY support USB C, but USB C supports ALL USB versions going back to USB 2.0. Now, depending on the age of your computer you will probably be stuck with a USB port that is pretty slow. Here is a chart to help you make sense of that:

With USB 2.0 things are further confused by the fact that there were different speeds of connections possible with 60MB/s being the maximum but slower connections being very common due to device limitations. The point being that your USB 2.0 device could be running at USB 1.0 speeds depending on whether the device, such as your card reader or USB hub, actually supports USB 2.0 High Speed.

There are two types of USB 3.1. USB 3.1 gen 1, and USB 3.1 gen 2. USB 3.1 gen 1 runs at the speeds shown above in the chart, but USB 3.1 gen 2 is a USB 3.2 speed protocol for USB 3.1 connectors. For instance, if you buy a USB A to USB C cable that says it is USB 3.1 gen 2 it will give you the USB 3.2 speeds.

Using the above chart you can see that any and all computers made before 2011 had USB 2.0 at best, so if your laptop or desktop is from 2011 or earlier this could be your reason for slow transfers.

The annoying thing is that even on new computers there are often still USB 2.0 ports. That is because many devices like a mouse and keyboard don’t need anything better than a USB 2.0 port. So, even though you might have a newer laptop a common theme I have seen is that a laptop might have 3 USB ports on it but 2 of them will be USB 2.0 and only 1 of them will be the newest USB 3.1 or USB 3.2 or whatever. If you are confused which one is which you can check your laptop case for a text label, try looking at the side or bottom of the laptop near the USB port in question, or you can also check your computer’s online user manual. If all else fails, try calling customer service.

Another common issue is that if you use an old USB 2.0 hub with a new USB 3.1 port you will only get the USB 2.0 speeds. So, if you are using a hub and getting slow speeds, try plugging your card reader directly into your computer and see if that improves things at all.

Of course the opposite is also true. If you plug a brand spanking new USB 3.2 hub into a USB 2.0 port, again, you only get the USB 2.0 speeds.

If you want the full speed of the USB port the card, the card reader, the cable, the USB hub, and the USB port on your computer all have to support the same maximum speeds.

Speaking of cables, this is yet another tricky situation with USB devices. Many companies are out there are selling non-standard cables, so watch out. As I said earlier USB C is backwards compatible with USB 2.0 and newer so just buying a USB C cable is no guarantee you will get the high speed transfers. You have to make sure that the USB cable you buy specifically supports the speeds you are trying to achieve.

Usually, devices and cables that support these standards will sport the “SuperSpeed” logo as a sign that they’re officially rated to achieve those speeds.

That just about covers the possibilities with USB devices and cables so on to the next possibility!

#3 Your card reader is slow

Card readers have to support all features of the USB connection and the SD card. You can’t just plug a UHS-III card into a card reader that only supports UHS-I. If you do that you will only get the UHS-I speeds at the most. The same is true for the USB support. If the reader only supports USB 3.0, you’ll be stuck with USB 3.0 speeds no matter what port you plug the card reader into.

#4 Your computer HDD is slow

There are literally dozens of things you have to know about how computers work just to transfer data from a card to a computer HDD.

Even if you have the fastest USB, the fastest USB cables, the fastest USB card reader, and the fastest CFExpress cards in the world, when all that data gets downloaded onto your computer it still has to be saved onto the HDD or SSD and if the storage device in your computer is slow guess what?? Your transfers will still be slow. The fact is that while a CFExpress card can manage about 1,700 MB/s read speeds and the USB 3.2 spec can support that, the fastest mechanical hard drives will generally write to disk at about 170 MB/s, or about 10 times slower than the fastest cards.

In order to fix that problem, you have to get a faster SSD in your computer. Typically, that is going to be an NVME SSD. The good news is that as of the writing of this article 8TB SSD drives are currently available. So, using SSDs for media storage is finally possible.

If you have an older laptop you may be limited to SATA 3, that interface is limited to 600 MB/s which is still fast enough for most transfers you might encounter. But, if you are trying to transfer terabytes of data from your cards 600 MB/s may not be fast enough. 600 MB/s works out to about 32 minutes to transfer 1TB if that helps you decide whether you need to buy a new NVME enabled laptop or desktop.

IF you have a desktop computer with free PCIE slots you can purchase an NVME expansion card for your computer.

#5 Transfers are still slow even though you know all of the above should be correct

In the case that you have verified all of the above should be correct to enable high speed transfers to your computer, there are rare situations where your drivers are incorrect. Usually Windows will automatically install the correct drivers but sometimes things get messed up. You also could try reinstalling your operating system.

Another unfortunate possibility is a hardware failure of some sort. A cable may be failing or a USB port may be failing on your computer. In those cases try another port, USB hub, or even another computer and see if you get different results. It may seem crazy, but I have had USB ports fail on my computer from overuse. They usually don’t stop working entirely. What I experienced is that the port would become intermittent during transfers, slowing down to just a few KB/s then speeding up for a few seconds before slowing down again.

I have learned from experience that many of these USB related products have questionable performance characteristics either due to buggy chipsets or just being sold as something they are not. Don’t automatically take your cable and USB hub’s performance claims 100% at face value. Test them out on a system you know works properly. This is especially true if you went cheap and got one of those weird Amazon brands for a lot less money than seems normal (I speak from experience).

If all else fails, call an expert and see if they can help you figure out where the issue may be.

#6 SD cards aren’t fast enough for your needs

The highest read speed for SD cards as of writing is 300MB/s. Future cards may be faster, or they may not. To get faster transfers to your PC you’ll need CFExpress Type B cards, or you can use an external recording device that uses CFExpress Type B or SSD (NVME) drives to save your video and photo data.

Troubleshooting:

Problem: Transfers start out fast then slow down.

Answer 1: The computer HDD may be too slow to sustain a large file transfer.

Answer 2: The USB port may be failing. One sign of a USB port failure is highly irregular transfer speeds or extremely slow transfer speeds that never get faster.

Answer 3: The SD card, computer SSD, or USB port may be overheating due to excessive use or other problems.

Problem: Transfers start out slow and stay slow.

Answer 1: Some part of the system is slowing everything down. See Above.

Problem: Transfer speeds go up and down repeatedly during a single transfer.

Answer 1: Most likely a throttling issue with an SSD or the SD card due to overheating.

Answer 2: The computer may be busy doing other tasks which are causing the transfer to slow down. Wait till the computer is done doing things then try again.