How to test microSD read speed through a USB 3 hub

Your microSD card may seem slow when you open photos or copy a video, even though it is connected through a USB 3 hub. The card, reader, hub, cable, or another transfer in progress can affect the result.

Use the USB & SD Speed Test to measure real read and write speed in your browser. Compare repeat runs, then test the reader directly on the computer to help find the bottleneck.

Test the microSD card through the hub

1. Connect the card reader to the USB 3 hub, insert the microSD card, then connect the hub to your computer. Use the same setup you want to check.
2. Open the USB & SD Speed Test. Follow the on-screen prompts to select the card or a test location on it. If your browser asks to access the card, allow access.
3. Run the read measurement and note the result and units. If the tool also measures write speed, record that separately. Read and write speeds are different measurements.
4. Repeat the read measurement three times without changing the setup. Let each run finish before starting the next one.
5. For a useful comparison, connect the same reader directly to a USB 3 port on the computer and repeat the test. Keep the card, reader, and test conditions the same.

Close any file copy or backup using the card before testing. If the tool asks to write test data, use the location it identifies for testing, wait for the run to finish, and do not remove the card during the test. Back up important files first.

What the result means

The three readings are close together

A consistent result suggests the setup is behaving consistently under those conditions. It does not prove the card reaches its advertised maximum speed. A browser test measures the card through the entire chain, including the card reader, hub, connection, and computer.

The read speed is lower through the hub

The hub, its cable, or another device using the hub may be limiting the connection. A reader or card can also be the slowest part. Compare with the same reader connected directly to the computer before blaming the card.

The readings change a lot between runs

Background copying, other devices sharing the hub, or the card's own behavior can cause variation. Repeat the test with transfers stopped and other devices unplugged from the hub. If the card becomes very hot or disconnects, stop testing and check the reader and card before continuing.

The result looks much lower than the card label

Check what the label actually describes. U1, U3, V30, and similar markings refer to minimum sustained write performance under specified conditions, not a guaranteed read speed through any reader. Also check the units: MB/s is megabytes per second, while Mbps is megabits per second. One MB/s equals 8 Mbps.

The browser cannot access the card

The computer may not recognize the reader, or the browser may not have access to the selected location. Remove and reinsert the card, reconnect the reader, and check whether the card appears in the computer's file browser. Do not format it just to make the test run if it contains files you need.

How to fix a slow result

1. Stop other transfers. Pause copying, backups, downloads to the card, and other activity on the hub. Run the test again with the card otherwise idle.
2. Check the USB 3 connection. Connect the hub to a USB 3 port and check that its upstream cable is fully seated. A USB 2 cable, port, or adapter can restrict the connection even when the hub has USB 3 ports.
3. Try the reader without the hub. Connect it directly to the computer, using the same port and card. If the result improves, test the hub's cable and try another hub port. Devices on a hub share its connection, so other busy devices can affect transfers.
4. Check that the computer sees the reader. In Windows 11, right-click Start and open Device Manager, then look under Disk drives and Universal Serial Bus controllers. On macOS, open Apple menu › About This Mac › More Info › System Report › USB. These screens can help confirm that the reader is detected, but a listed device alone does not prove the speed of the complete setup.
5. Reseat the card and inspect the reader. Remove the card, check that it is inserted in the correct direction, then try it again. If the reader has a removable microSD adapter, check that adapter too. Avoid scraping the contacts or applying liquid.
6. Compare another reader or card. Test the same card in a known-good reader, then try another card in the original reader. If only one card remains slow across readers, back up its files and consider replacing it. If several cards are slow in one reader, the reader is the more likely limit.

For more detail on why the reader matters, see Why card reader speed affects SD card test results.

When the card or reader may be faulty

A low speed number by itself does not prove damage. Look for stronger signs: repeated read errors, files that fail to open, the card disconnecting, or speed collapsing across different readers. Copy important files to another storage device as soon as you can. If errors follow the card, stop relying on it and replace it. If they follow the reader, replace the reader or have it checked.

A speed test measures performance, not the card's full capacity or the authenticity of its label. Use a dedicated capacity check before trusting an unfamiliar card with important files.

Why a USB 3 hub does not guarantee a fast read

The USB 3 rating describes the hub's connection capability, not the speed of every device attached to it. The microSD card, card reader, hub cable, computer port, and activity from other connected devices all affect the result. The slowest part can set the pace.

The USB & SD Speed Test runs in your browser, and files stay on your device.