QLC SSD Slows Down After the SLC Cache Runs Out

Updated on Nov 8, 2026 | by Qiling

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Why QLC Drives Slow Down
Measure the Drop First
Reduce the Impact
Clone to a Faster SSD
Frequently Asked Questions

A QLC SSD that copies at 1,000 MB/s for the first minute and then falls to 150 MB/s is not necessarily broken. It has exhausted the SLC cache, and the drive is now writing at its native four-bit-per-cell speed. Understanding that curve prevents you from replacing a healthy drive or, worse, interrupting a backup that only needed more time.

Why QLC Drives Slow Down

QLC NAND stores four bits in each cell, which makes it cheaper per terabyte but slower to program and less durable than TLC. To hide that, the controller writes incoming data first into a portion of the NAND treated as single-level cache. Bursts, small files, and ordinary desktop work stay inside that cache and feel fast.

When the cache fills, the controller must fold the cached data into QLC cells and accept new writes at the slower native rate. A dynamic cache also shrinks as the drive fills, so the same task that ran at full speed on an empty drive slows down much sooner at 80 percent capacity.

Long sequential writes expose the behavior: cloning a disk, copying a large game library, recording video, or restoring a backup. Short benchmarks often miss it because the test ends before the cache runs out.

Measure the Drop First

Watch the transfer rate during a sustained copy of at least 20 GB. A drive that starts fast and settles at a stable lower speed is behaving normally for QLC; a drive that drops to a few megabytes per second, stalls, or disconnects has a different problem, such as overheating, a failing cable, or a controller fault.

Check the drive temperature and the SMART health at the same time. QLC drives can throttle from heat as well as from cache exhaustion, and the fix for one does not solve the other.

Confirm how full the drive is. Below 50 percent usage the cache has room to work; above 85 percent both the cache and the background garbage collection have very little space, and even read performance can suffer.

Reduce the Impact

Keep at least 15 to 25 percent of the drive free. Let the drive sit idle after a large write so garbage collection and cache folding can finish; a laptop left on the charger overnight recovers much of its burst speed.

Enable TRIM, keep the firmware current, and avoid running two large writes at once. Do not defragment an SSD; it adds writes without improving performance. For a desktop, a simple heatsink or a airflow change can stop thermal throttling that makes the cache curve look worse.

If your work is dominated by sustained writes, such as video editing or daily disk images, plan for a TLC drive with a larger cache. QLC is a good fit for read-heavy storage and games, not for a busy write target.

Clone to a Faster SSD with Qiling

Qiling Disk Master can clone the system from a QLC drive to a TLC drive without reinstalling Windows. Clone during an idle period and let the task finish; a slow final phase is normal when the source cache is exhausted.

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Step 1. Check the Used Space

Confirm that the destination is at least as large as the used space on the source. If you are moving to a smaller SSD, shrink or move data first and leave room for the boot partitions.

Check the source disk before cloning from a QLC SSD

Step 2. Select Source and Destination

Choose Clone > Disk Clone, select the QLC drive as the source, then the new TLC drive as the destination. Match the drives by capacity and model, not by drive letter.

Select the faster TLC SSD as the clone destination

Step 3. Preview and Proceed

Review the layout and the SSD alignment option, then click Proceed. Boot from the new drive only after the task completes and the old drive has been disconnected.

Review the clone layout and click Proceed

Frequently Asked Questions

Is a speed drop normal for QLC?

Yes, once the SLC cache is full and the drive writes in QLC mode. The drop should be gradual and stable, not a stall or a disconnect.

Can I disable the SLC cache?

No. The cache is managed by the drive firmware. You can only leave it room to work by keeping free space and letting the drive idle.

Does more free space restore the speed?

It restores the cache size and gives garbage collection room, which usually brings back the burst speed.

Should I clone or reinstall?

Clone when you want to keep applications and settings. Reinstall when the old system is already unstable or full of problems.

For the cloning basics, see best free SSD cloning software.

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Windows 11/10/8/7, 100% Secure