Why USB drive performance drops as it fills

Your USB drive copied the first files quickly, but later transfers crawl. That can happen as free space runs low, though heat, the USB port, and the files themselves can also change the result.

Compare the drive at different fill levels before deciding it is faulty. The USB & SD Speed Test measures read and write speed on a flash drive or memory card, so you can check whether the slowdown follows the drive or the conditions around it.

Test the drive at different fill levels

Back up important files first. Open the USB & SD Speed Test and choose the USB drive for testing if prompted.
Start with the drive mostly empty. Use the same computer and USB port for each measurement, and pause file copies or backups while testing.
Run the test and note the read and write results, along with how much free space the drive has. Read speed measures how quickly data comes off the drive; write speed measures how quickly it goes on.
Copy ordinary files to the drive until it has less free space, but do not fill it completely. Check the available space in Windows 11 under Settings › System › Storage, or on a Mac under Apple menu › System Settings › General › Storage.
Run the same test again. If possible, repeat it after the drive has cooled, using the same computer, port, and test conditions.

Do not use a drive containing the only copy of important files for repeated write tests. A speed result is a performance measurement, not a data-integrity or capacity check.

What the result means

Write speed falls as free space gets low

This pattern can be normal. Some flash drives use a fast area of memory as a temporary write cache. When that area fills, later writes may go directly to slower flash memory, while the drive also reorganizes stored data to make room for new writes. With less free space, that work can become more noticeable.

The size of the cache and the speed of the underlying memory vary by drive. The change may be slight on one model and dramatic on another. A slowdown does not, by itself, prove that the drive is fake or failing.

Read speed stays similar, but write speed drops

That points more toward the drive's write behavior, cache, or low free space than a general connection problem. Compare results at similar fill levels and make sure no other program is copying files to the drive at the same time.

Both read and write speed fall

Check for heat, a busy computer, or a connection issue. A drive can slow during a long transfer if it gets warm. A hub, adapter, or slower USB port can also limit both directions.

The test stays fast, but copying files is slow

The copy itself may be the reason. A folder of thousands of small files takes longer than one large file, even when the total size is similar. The computer may also be scanning files, or the source drive may be slow. Try a large file and a folder of small files separately to compare.

How to reduce the slowdown

Leave some space free. Move files you do not need on the USB drive to another location. There is no single free-space percentage that guarantees full speed, but a nearly full drive often has less room to manage new writes.
Use a direct USB port. Connect the drive to the computer instead of a hub or extension, if possible. Try another port and repeat the test. A port that looks the same may still use a different connection speed.
Pause other transfers. Stop backups, file copies, and applications that are writing to the drive. Then test again so the result reflects one workload at a time.
Let the drive cool. If it is warm after a long copy, safely eject it, unplug it, and let it cool before testing again. Compare the result with a short test after a cool start.
Compare a large file with many small files. If only the folder of small files crawls, the number of separate file operations may be the main issue. Archive the folder into one file before copying when that suits your workflow.
Test on another computer. Use the same drive with a different computer and, if practical, a different port. If the slowdown follows the drive when it is mostly empty and cool, the drive itself is a stronger suspect.

When the drive may be failing

A gradual write-speed drop near full capacity can be a design limitation. Be more concerned if the drive becomes extremely slow while mostly empty, disconnects during ordinary transfers, reports errors, or files disappear or become corrupted. Copy important data off it before further testing.

If the same problems happen on another computer and port, replace the drive or ask the seller about warranty service. Do not keep using a drive with repeated file errors for the only copy of anything important.

Why the first copy can look much faster

A short transfer often fits inside the drive's fast cache, so it shows burst speed rather than the speed the drive can maintain. A longer transfer may outlast that cache and reveal a lower sustained rate. The free-space level, transfer length, temperature, and file sizes all affect what you see.

For a useful comparison, keep the test conditions the same and record both the free space and the read and write results. Treat one unusual run as a clue, not a final verdict.