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Why a USB-C connector doesn't guarantee high speed

Learn why USB-C ports and cables can still be slow, how to test a flash drive or card, and which part of the connection limits transfers.

Daniel HayesHardware tests Tests 4 min read

Your flash drive has a USB-C plug, but copying a file still takes ages. The connector shape alone does not tell you how fast the drive can transfer data. The port, cable, drive, and any hub in between all affect the result.

Test the drive first, then check each part of the connection. TooLab’s USB & SD Speed Test measures the real read and write speed of a flash drive or memory card.

Test the drive's transfer speed

  1. Open the USB & SD Speed Test and follow the on-screen prompt to test your flash drive or memory card.
  2. Connect the drive directly to the computer, not through a hub or dock. If your computer has more than one USB-C port, note which port you use.
  3. Let the read and write measurements finish. Write speed is how quickly the drive saves data. Read speed is how quickly it retrieves data.
  4. Repeat the test in another USB-C port, if available. Keep the same drive and cable so you can compare the port results.
  5. If you normally use a hub or adapter, test that setup separately. Compare it with the direct connection to see whether the hub changes the result.

Close other transfers while testing. A drive copying files in the background, or several devices sharing one hub, can affect the measurement.

What the result means

The drive is slow in every port

The drive itself may be the limit. Some compact flash drives and memory cards have modest write speeds even when their connector and the computer's port support faster USB modes. Read and write speeds can also differ: a drive may read files quickly but save them slowly.

The drive is faster in one port

The slower port may support an older USB data standard, or it may share bandwidth with other connections. The USB-C connector looks the same either way. Check the computer's manual or specifications for the data speed of that exact port.

It is slower through a hub or adapter

The hub, its cable, or another connected device may be limiting the connection. A hub can share bandwidth between several devices, and an adapter can support a slower data standard than the computer's port.

The test is faster than copying your usual files

Real file copies do not always match a speed test. Many small files take longer to copy than one large file because the drive has to handle each file separately. The file system, available space, and background activity can also change copy speed.

How to get the speed your devices support

  1. Connect the drive directly. Remove the hub, dock, and adapter for a comparison. If speed improves, reconnect those accessories one at a time to find which one makes the transfer slower.
  2. Check the cable's data rating. USB-C cables can look identical but support different data speeds. A cable advertised for high-wattage charging may still have USB 2.0 data capability, or may be intended for charging only. Check the cable's specifications rather than judging by its shape.
  3. Look up the exact port specification. Check the computer manufacturer's specifications for the model and port you are using. A USB-C port can support USB 2.0 data, a faster USB standard, or another capability such as USB4. USB-C describes the connector, not the speed.
  4. Check the drive's own rating. Compare its advertised read and write speeds with your test results. The drive cannot transfer faster than its own controller and memory allow, even when the port and cable are faster.
  5. Try another port and cable. Keep the drive the same while you swap one part at a time. This helps separate a slow drive from a slow cable or port. On a Mac, open Apple menu › About This Mac › More Info › System Report › USB to inspect connected USB devices and any speed information shown. Windows does not provide a simple built-in screen that reliably identifies the negotiated data speed for every USB connection, so check the device specifications and compare actual results.
  6. Check for a loose or damaged connection. Try another cable if the drive disconnects, transfers at changing speeds, or responds when you move the plug. Do not force a connector into the port.

Why the USB-C shape can be misleading

USB-C is the name of a small, reversible connector. It does not promise a particular data rate. USB 2.0, for example, has a theoretical maximum of 480 megabits per second, which is 60 megabytes per second before overhead. USB 3.2 Gen 1 has a nominal rate of 5 gigabits per second, but that is not a guarantee that files will copy at the same rate. The slowest part of the connection sets the practical limit.

Charging and data transfer are separate capabilities. A cable or port can charge a device quickly while transferring data slowly. Likewise, support for a faster USB mode must be present in the computer port, cable, and drive. If even one part supports only a slower mode, the connection uses the slower option.

When comparing speeds, check whether the number is in megabits per second (Mbps) or megabytes per second (MB/s). One byte contains eight bits, so the units are not interchangeable. The USB & SD Speed Test reports the drive's actual performance, which is more useful than the connector shape alone.

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