You copy photos from an SD card to your laptop and see:

80–90 MB/s

Everything looks normal.

Then you copy a large video file back to the same card.

Write speed drops to:

20 MB/s

10 MB/s

or even lower.

Because the card is connected through a USB-C hub, it is easy to conclude:

“The hub can read fast but cannot write fast.”

Sometimes the reader path is the limit.

But fast reads and slow writes can also be completely normal for a card whose advertised maximum read speed is much higher than its sustained write capability.

The best way to diagnose the problem is to compare four numbers:

Advertised Read Speed → Speed Class → Measured Read → Measured Write

Those four numbers usually tell you whether the bottleneck is the card, reader mode, workload, or USB-C connection.

Start With Four Numbers, Not the Hub

Before changing ports, record this:

At first glance:

88 MB/s read vs. 32 MB/s write

looks like something is wrong.

But a V30 card guarantees a defined minimum sequential write performance of 30 MB/s under the relevant standard conditions—it does not mean its write speed should equal its advertised maximum read speed.

The SD Association's Speed Class standards for SD cards distinguish Speed Class, UHS Speed Class, and Video Speed Class as minimum sequential performance ratings.

That distinction solves many “fast read, slow write” cases before you touch the USB-C hub.

Decode the Symbols on the SD Card

Look at the front of the card.

You may see symbols such as:

C10

U1

U3

V10

V30

V60

V90

These numbers are particularly useful when diagnosing write performance.

The SD Association emphasizes that these classes describe assured minimum sequential access performance under defined conditions, which is especially important for continuous recording workloads.

So a package can advertise:

“Up to 160 MB/s Read”

while the card itself carries:

V30

Those two numbers describe different things.

Do not compare them as though they were supposed to match.

Read the Speed Gap

Once you know the card class, your measured read/write result becomes much easier to interpret.

Pattern A — Fast Read, Write Near the Card's Rated Minimum

Example:

Read: 90 MB/s

Write: 32 MB/s

Card: V30

That result may be completely reasonable.

The card is reading close to the UHS-I reader's upper range while maintaining write performance around its guaranteed class.

The USB-C hub is not automatically the bottleneck.

Pattern B — Both Read and Write Stop Around 20–25 MB/s

This suggests a different problem.

The card and reader may be operating in an older High Speed bus mode rather than a faster UHS mode.

The SD Association's SD bus interface specifications list theoretical interface ceilings including approximately:

High Speed → 25 MB/s

UHS-I → up to 104 MB/s

UHS-II → up to 312 MB/s in supported modes

while also noting that actual card performance can be lower than the bus-interface maximum.

If both directions consistently cap near the same low range, investigate the reader mode and card/reader compatibility.

Pattern C — UHS-II Card, but Both Directions Stop Around UHS-I Speeds

Check the card's contacts.

UHS-II cards have a second row of contacts.

If you put a fast UHS-II card into a UHS-I reader, it can still work because the standards are designed with backward compatibility—but it cannot use the full UHS-II interface through that reader.

So a card advertised above 200 MB/s may suddenly perform much closer to UHS-I territory.

That does not necessarily mean:

Card failure

or:

USB-C hub failure.

It may simply mean:

UHS-II Card + UHS-I Reader = UHS-I Connection Mode

If your workflow genuinely needs V60/V90 or very high offload/write performance, verify that both the card and reader support the required UHS interface.

Use One Large File Before Testing Hundreds of Photos

Another trap is comparing two completely different workloads.

For example:

Read Test

Copy:

one 8GB video

from:

SD → Internal SSD

Write Test

Copy:

4,000 JPEGs

from:

Laptop → SD

Those are not equivalent tests.

Thousands of small files require repeated:

• file creation;
• metadata updates;
• directory updates;
• allocation operations.

That can produce much lower observed throughput than writing one large sequential file.

For the first controlled test, use:

one large file

in both directions.

For example:

SD → Internal SSD

then:

Internal SSD → SD

Record the average speed.

This gives you a much cleaner read/write comparison.

Do Not Benchmark Card-to-Drive Through the Same Hub First

Suppose you test:

SD Card → USB-C Hub → External SSD

The reader and destination drive are now both using the hub.

If the transfer is slow, you no longer know whether the limit is:

SD card

card reader

shared USB path

external SSD

or:

destination file system

For the cleanest first benchmark, use the laptop's internal SSD as the destination/source.

Test:

SD → Internal SSD

for card read speed.

Then:

Internal SSD → SD

for card write speed.

Only after those numbers make sense should you test:

SD → External SSD

or:

External SSD → SD

through the same hub.

This also keeps the new article separate from the broader USB-C hub slow data transfer troubleshooting guide, which focuses on general USB throughput rather than the specific read/write asymmetry of an SD card.

Check Whether the Card Is Nearly Full

Flash storage can behave differently when little free space remains.

If the SD card is almost full, first back up its contents.

Then compare performance with more free space available.

The SD Association also notes that repeated deletion and writing can create fragmentation, and fragmented areas can reduce write performance compared with clean sequential writes.

Do not format a card containing important files just to run a speed test.

But after the files are safely backed up, formatting the card in the original camera or intended device can sometimes restore a cleaner recording layout.

Always use the file system and formatting method recommended by the device manufacturer.

If the Card Writes Slowly in Every Reader, Stop Blaming the Hub

Now test the same card using another known-good compatible reader.

Compare:

Card → USB-C Hub Reader

with:

Card → Second Reader

If the write speed remains similarly low through both readers, the strongest suspect becomes:

the card itself

rather than the hub.

This is especially convincing if:

• read speed remains high;
• write speed is consistently low;
• the same behavior appears on another computer;
• multiple large-file tests produce similar results.

Also test another SD card in the original hub.

You now have four useful combinations:

This is much more useful than replacing the hub based on one benchmark.

When the Reader Mode Is Actually the Bottleneck

The USB-C hub becomes more relevant when:

The same card writes significantly faster in another reader

while:

the hub's card slot consistently caps both read and write performance.

At that point, check the actual specification of the card reader inside the hub.

Look for:

SD 2.0

SD 3.0

UHS-I

UHS-II

and the specified maximum reader rate.

Do not judge the card reader by the USB-C connector.

A hub can have:

5Gbps USB 3.0 ports

while its internal SD reader is still:

UHS-I

with a much lower card-interface ceiling.

The fastest interface number printed anywhere on the hub does not automatically apply to every built-in port.

Match the Reader to the Card You Actually Use

For photographers or creators working primarily with UHS-I SD and microSD cards, a USB-C hub with UHS-I SD/Micro SD reading up to 104 MB/s, three USB 3.0 ports, and 100W PD pass-through provides a practical mixed-media workflow.

The LENTION CB-C36B supports simultaneous SD and microSD operation at up to its specified UHS-I reader rate, while the USB-A ports provide additional connections for external storage and ordinary peripherals. Its PD input also allows the laptop to remain powered while the card workflow is active.

This is appropriate when your workflow looks like:

UHS-I SD Card → Hub → Laptop

alongside:

External SSD / Mouse / Keyboard

and:

PD Charger

But it is important not to overstate what the specification means:

A UHS-I reader rated up to 104 MB/s does not guarantee 104 MB/s write speed from every SD card.

The card's own controller, NAND, Speed Class, workload, and free-space condition still influence write performance.

For a fixed creator desk where cable placement matters, a long-cable USB-C hub with UHS-I SD/Micro SD slots, USB-A/USB-C data ports, and charging pass-through offers another layout option.

The LENTION CB-C37-1M uses a 3.3-foot host cable and specifies UHS-I card transfer up to 95 MB/s alongside USB-A 3.0, USB-C data, HDMI, audio, and a separate charging port.

That makes it more suitable for a fixed photo/video workstation where the hub needs to sit away from the laptop.

Again, neither model should be recommended as a way to unlock UHS-II performance.

If you use a high-end UHS-II card and need its full speed, use a reader that explicitly supports UHS-II.

Do Not Confuse “Slow Write” With “Card Not Working”

This article deals with a card that:

mounts normally

reads normally

but:

writes much more slowly than expected.

That is different from several other failure states.

If the card does not appear at all, use the SD card not showing through a USB-C hub guide.

If Windows asks you to format the card, do not treat that as a speed problem. Follow the SD card needs formatting troubleshooting workflow and protect important files before modifying the card.

And if your problem is that SD and microSD work individually but not together, the relevant variable is reader design rather than write speed. The SD and microSD simultaneous-reading guide covers that separate behavior.

Read Your Numbers Like This

Once you finish testing, your result should fit one of these patterns.

Read Fast + Write Meets Speed Class

Example:

Read 90 MB/s

Write 32 MB/s

V30 card

→ Write speed may be normal for the card.

Read and Write Both Around 20–25 MB/s

→ Check whether the reader is operating below UHS-I mode.

UHS-II Card + UHS-I Reader

→ Reader mode is limiting the card.

Use a UHS-II reader if the higher interface speed is required.

Large File Writes Fast, Small Files Write Slowly

→ Workload is affecting measured throughput.

Do not compare sequential performance with thousands of small files.

Same Card Writes Slowly in Every Reader

→ Investigate the card itself.

Same Card Writes Normally Elsewhere but Slowly Through One Hub

→ Investigate that reader mode, hub path, or simultaneous workload.

Write Speed Drops Only When Other Hub Devices Are Active

→ Reduce simultaneous activity and test the SD card alone before concluding that the card is slow.

Quick Answers

Why can my SD card read at 90 MB/s but write at only 30 MB/s?

Read and write performance do not have to be equal. The card's advertised maximum read speed and its guaranteed Speed Class write performance describe different limits.

What does V30 mean?

V30 indicates a Video Speed Class with at least 30 MB/s minimum sequential performance under the SD standard's defined conditions.

Does UHS-I mean my card will write at 104 MB/s?

No. 104 MB/s is an upper bus-interface rate for supported UHS-I modes, not a guaranteed write speed for every card.

Why is my UHS-II card slow in my USB-C hub?

If the hub's reader supports only UHS-I, the card cannot use its full UHS-II interface through that reader.

Why do thousands of photos copy more slowly than one video file?

Small-file transfers require many separate file and metadata operations, so they usually do not behave like one continuous sequential write.

Does Power Delivery make an SD card write faster?

Not directly. Power can help keep a multi-device hub setup stable, but it does not increase the SD card's own write-speed capability.

Final Takeaway

When an SD card reads fast but writes slowly through a USB-C hub, do not compare only:

90 MB/s read

vs.

30 MB/s write

and conclude that something is broken.

Compare:

Advertised Read Speed

→ Speed Class

→ Reader Mode

→ Measured Read

→ Measured Write

→ File Workload

The most important rule is:

The maximum read speed printed on an SD card is not the same thing as its guaranteed sustained write speed.

And when the USB-C hub is involved:

The reader can only expose the SD bus mode it supports; a UHS-II card connected through a UHS-I reader remains limited by the UHS-I reader path.

Once you separate card capability from reader capability, a large read/write gap becomes much easier to explain—and you can decide whether you actually need a different card, a different reader, or simply a more realistic benchmark.