SSD slower than advertised? Why speed tests and big copies fall short
The box says 7,000 MB/s. CrystalDiskMark says 3,500, and a 100 GB copy crawls along at 400. Usually the drive is fine and something around it is the limit.
What the box number means
"Up to 7,000 MB/s" is a sequential read, in big chunks, on a fast PCIe 4.0 x4 slot, into a drive that's empty and cool. Writes are usually lower, small random files are far lower, and everyday tasks like booting or loading games rarely feel different between a 3,500 and a 7,000 MB/s drive. A big gap is still worth checking, because it often has a simple cause.
Common causes, from most to least likely
| Cause | Typical result | How to check |
|---|---|---|
| M.2 slot is PCIe 3.0 (older boards, some second slots) | Tops out around 3,500 MB/s | Motherboard manual; CrystalDiskInfo "Transfer Mode" shows current | supported, e.g. PCIe 3.0 x4 | PCIe 4.0 x4 |
| Slot or adapter runs at x2, or lanes shared with a SATA port or GPU slot | About half the expected speed | Transfer Mode shows x2; manual's lane-sharing table |
| SATA M.2 drive, or an NVMe drive in a SATA-only slot (it won't be detected) | About 550 MB/s maximum | Drive spec says SATA; slot labelled SATA |
| USB enclosure or port | USB 3.2 Gen 1 ~400-450, Gen 2 ~900-1,000, Gen 2x2 ~2,000, USB4/Thunderbolt ~3,000-3,800 MB/s | Which port and cable you used; front-panel ports are often slower |
| SLC cache runs out on big writes | Fast for the first few to few hundred GB, then a sudden drop | A long copy graph that falls off a cliff and stays low |
| QLC or DRAM-less drive | Very low after the cache (sometimes under 200 MB/s); slower random access | Drive model and reviews |
| Thermal throttling | Speed falls after a minute of heavy use, recovers when idle | Temperature in CrystalDiskInfo or smartctl; above about 70-80 °C under load is hot |
| Drive nearly full | Smaller cache, slower writes | Keep 10-20% free |
| TRIM off, old firmware, power saving | Gradual slowdown, odd stalls | fsutil behavior query DisableDeleteNotify should be 0; maker's tool for firmware |
Testing it properly
- Check the link in CrystalDiskInfo (Windows), System Information > NVMExpress (Mac) or
sudo lspci -vv"LnkSta" (Linux). The current speed and width should match the drive and slot. - Run a benchmark: CrystalDiskMark (Windows), KDiskMark or
fio(Linux), Blackmagic Disk Speed Test or AmorphousDiskMark (Mac). Use the default 1 GiB test; the "SEQ1M Q8T1" line is the one comparable with the box figure. - Mind the other end. A copy is only as fast as the slower drive: copying from a hard disk (about 100-250 MB/s) or a USB stick limits it no matter how fast the SSD is, and lots of small files are always slow.
- Check health while you're there. Wear, spare blocks and media errors are covered in SSD health percentage, and the SMART report explainer reads the raw output for you.
Fixes
- Move the drive to the CPU-connected M.2 slot (usually the top one) and check the manual's lane-sharing notes.
- Use the heatsink that came with the board, or a cheap M.2 heatsink, and make sure there's airflow.
- Keep 10-20% free, and keep TRIM on. After cloning from an old drive, see clone to SSD for the TRIM check and partition steps.
- For an external SSD, use a rear USB port with the right standard and the cable that came with it; see USB transfer speed drops.
- Update the drive firmware with the maker's tool.
- If big sustained writes matter to you (video, large game libraries), choose a TLC drive with DRAM next time; QLC is fine for reading but slow for long writes.
Related
Clone a drive to SSD · New SSD not showing up · SSD health percentage · Fake USB stick or SD card test
Questions
Why is my NVMe SSD only getting about 3,500 MB/s?
It is almost certainly in a PCIe 3.0 slot, which tops out around 3,500 MB/s. Check the Transfer Mode in CrystalDiskInfo and the motherboard manual for a PCIe 4.0 slot.
Why does my SSD slow down during large file copies?
Most consumer SSDs write first to a fast SLC cache. Once a big copy fills it, speed drops to the native speed, which on QLC or DRAM-less drives can be a few hundred MB/s. Heat can add to it.
Is CrystalDiskMark's result accurate?
Yes for comparison. The SEQ1M Q8T1 figure is the one to compare with the advertised speed; random 4K figures are always much lower.
Does a full SSD get slower?
Yes. A nearly full drive has less room for its cache and spare blocks, so writes slow down. Keep about 10-20% free.
Why is my external SSD only 400 MB/s?
That is the limit of a USB 3.2 Gen 1 port or cable. A Gen 2 port gives about 1,000 MB/s and USB4 or Thunderbolt about 3,000 MB/s or more, if the enclosure supports it.
Published .