Why SSD speed drops during thermal throttling

Your SSD starts a file transfer quickly, then slows down after a few minutes. This can happen when the drive gets hot, but temperature is not the only possible cause. A write cache filling up, a busy computer, or a slow cable or enclosure can produce a similar drop.

Use the Storage Test to compare large-file and small-write performance under consistent conditions. It cannot read an SSD's temperature, but repeating the test after the drive cools can help you see whether heat is part of the pattern.

Compare SSD speed when cold and warm

1. Open the Storage Test on the computer and drive you want to check. Keep important files backed up, and close apps that may be copying, downloading, or syncing files.
2. Run the test once after the drive has been idle for a while. Note the large-file read and write speeds, plus the small-write result.
3. Use the drive for a normal, sustained task that usually causes the slowdown, such as copying a large file. Do not create an unusually heavy workload just to heat it up.
4. Run the same test again soon afterward, without changing the connection or power settings. Compare the results with the first run.
5. Let the drive sit idle and cool, then repeat the test once more. Look for a pattern across runs rather than treating one score as proof.

Keep the drive connected to the same port, cable, and hub arrangement for every run. If you change several things at once, you will not know which one affected the result.

What the result means

Speed falls when warm and comes back after cooling

This pattern is consistent with heat-related slowdown. An SSD controller can reduce performance when its temperature rises to protect the drive. The test cannot confirm the temperature or prove that the controller is throttling, so treat the repeatable change as a clue, not a diagnosis.

Write speed drops, but read speed stays closer to normal

Heat may be involved, but SSDs can also write quickly to a temporary cache and slow down after that cache fills. This is common during long writes, especially on some consumer drives. If the same drop happens at a similar point even when the drive starts cool, cache behavior may explain more than temperature.

Results are low in every run

A low result that does not improve after cooling is less suggestive of thermal throttling by itself. The drive may be limited by its connection, the computer, or the workload. A browser test is also not a direct measurement of every application’s file-copy performance.

Only an external SSD slows down

Check the whole connection path. A USB hub, dock, cable, or enclosure can limit speed or add heat. The drive inside an enclosure may also get hotter than its outer case suggests.

How to reduce heat-related slowdowns

1. Pause the workload and let the drive cool. Stop large copies, downloads, backups, or game installs and wait before testing again. Do not put a drive in a refrigerator or freezer, since condensation can damage electronics.
2. Give the drive room for airflow. Place an external SSD on a firm, open surface instead of a blanket or inside a closed drawer. Keep a laptop’s vents clear. Do not remove an enclosure or laptop cover unless you know how to do so safely.
3. Connect the drive directly. For an external SSD, try a suitable port on the computer instead of a hub or dock, and check the cable for damage. Keep the same direct connection when comparing results.
4. Check for other disk activity. On Windows 11, open Task Manager with Ctrl + Shift + Esc and look for apps using the disk. On macOS, open Activity Monitor and check the Disk view. Pause unnecessary syncing or copying, then retest after activity settles.
5. Check the power mode. On Windows 11, open Settings › System › Power & battery and check the power mode. On macOS, open System Settings › Battery and check whether Low Power Mode is enabled. Power settings can affect performance, so use the same setting for each comparison.
6. Check drive and enclosure support information. If the slowdown continues, consult the drive or computer manufacturer’s guidance for temperature monitoring, firmware, and cooling. Back up important files before changing firmware or hardware.

When the SSD may need service

One speed drop does not mean the drive is failing. If results remain unusually low after the drive cools, the connection is direct, and background disk activity has stopped, test the drive on another compatible computer if possible. A slowdown that follows an external enclosure may point to the enclosure or cable, not the SSD itself.

Back up important data if the drive disconnects, produces read or write errors, or becomes unreliable, then contact the manufacturer about warranty or service. For an internal drive, have a repair shop check it if you cannot safely remove or test it yourself. A browser benchmark can show a change in performance, but it cannot diagnose a damaged controller or memory chips.

Why the browser test cannot confirm temperature

A browser-based storage test measures how quickly test data can be read or written. It does not have access to every drive’s temperature sensors, and it cannot identify the exact cause of a slowdown. Temperature readings, when available, usually come from drive-specific monitoring software or system tools supported by the hardware.

The Storage Test uses temporary data rather than your personal files. For more on what browser storage tests do, see why browser storage tests use temporary data. If your results collapse during a run, check the other causes of a storage speed collapse before deciding the SSD is overheating.