Why USB controllers affect sustained throughput

A USB drive may start copying quickly, then slow down, especially when other devices are plugged in. The USB controller, the hub, and the drive itself can all affect how fast data keeps moving.

Use the USB & SD Speed Test to compare read and write speeds, then check whether a port, shared connection, or the drive is the likely limit.

Compare USB speeds fairly

1. Open the USB & SD Speed Test and follow the prompts to measure the USB drive or memory card. Before a write test, back up important files and follow any on-screen instructions about test data.
2. Note the read and write results, and which port or hub the drive is connected to. Keep the drive, computer, and test conditions the same for comparisons.
3. Repeat the test with the drive connected directly to a different USB port, without a hub or dock. This helps show whether the first port or its connection is limiting speed.
4. Run one more comparison while other high-traffic USB devices are disconnected. If speed improves, reconnect them one at a time to identify which setup changes the result.

Do not compare a test with a quiet computer to one during a large file copy or backup. Background transfers can use the same connection and make the drive appear slower than it is on its own.

What the result means

The drive is faster when connected directly

The hub, dock, cable, or its upstream connection may be the bottleneck. A hub's devices share the connection back to the computer, so several active devices can compete for its available bandwidth. A different port may also connect through a different USB controller.

Several ports show similar speeds

The drive may be the limit, or the ports may share the same controller and internal connection. Similar results do not prove the controller is faulty. Many flash drives have much lower real-world speeds than the maximum signaling rate printed on a USB port.

Read speed is much higher than write speed

This is common. Flash memory can often read data faster than it can write it, and a drive may use a small fast cache that fills during a long transfer. A short test can look quick even when a large copy later slows down.

Speed falls when other USB devices are active

The devices may be sharing a hub, controller, or another part of the computer's internal data path. The result can also change if another device is using storage or if the computer is busy. Test with the other device unplugged, then try a port on a different side of the computer if available.

Results vary between runs

Small changes are normal. The operating system may be doing background work, and flash drives can change speed as their cache fills or they warm up. Compare repeated runs under similar conditions instead of treating one result as a fixed rating.

How to reduce USB slowdowns

1. Connect the drive directly. Remove the hub, dock, extension cable, or adapter for a comparison. On a desktop, try a rear motherboard port as well as a front-panel port, since front ports often connect through extra wiring.
2. Check the port and cable rating. A USB-C connector does not guarantee a particular data speed. Check the computer and cable specifications, and try a known high-speed cable for an external SSD. A USB 2 connection can limit a faster drive even when the drive itself supports a newer standard.
3. Separate busy devices. Disconnect other drives, cameras, or capture devices while testing. If you need them connected, place them on another hub or port and compare again. A different physical port does not always mean a different controller, but it is a useful test.
4. Check which USB controller the system sees. In Windows 11, right-click Start, open Device Manager, and expand Universal Serial Bus controllers. On macOS, open Apple menu › About This Mac › More Info › System Report, then select USB. These lists can show connected devices and bus information, but they may not make the computer's port layout obvious.
5. Install system updates. In Windows 11, open Settings › Windows Update and install available updates. On macOS, open System Settings › General › Software Update. If a manufacturer provides a chipset or USB driver for your computer, use its official support instructions rather than a driver download from an unknown source.
6. Check for heat or a failing connection. If the drive slows after several minutes, feels unusually hot, or disconnects when moved, test another port and cable. Stop using a drive that repeatedly disconnects during writes until important files are backed up.

Why a USB controller matters

A USB host controller manages communication between the computer and connected USB devices. It also connects to USB hubs, which split one upstream connection into several ports. When several devices send data at once, they can compete for bandwidth on a shared path. The exact layout differs by computer, so two ports next to each other might share a controller, while ports on different sides might not.

USB standards list maximum signaling rates, not guaranteed file-copy speeds. For example, USB 2.0 High-Speed has a theoretical rate of 480 megabits per second, while many USB 3.x Gen 1 ports are rated at 5 gigabits per second. Protocol overhead, the drive's memory, its controller, the cable, and the computer's storage system all reduce practical throughput. A faster port can help only if the rest of the path can use it.

A browser-based speed test is useful for comparing ports and devices, but a short run may not represent a long transfer of many gigabytes. If speed drops during a real copy, compare the same large file in the same conditions, and watch whether the slowdown happens with one port, one hub, or one drive. That pattern is more useful than the port's label alone.