Why Cheap SSDs Slow Down on Large File Transfers

Cheap SSDs slow down during large file transfers because they lack a large, fast SLC write cache and use lower-quality NAND flash, which forces the drive to stop and erase data in the background while you are still writing. When you copy a multi-gigabyte file, the drive’s controller first writes to a small high-speed cache; once that fills, it must directly write to the slower NAND, and the speed drops dramatically. This article explains the exact mechanics, how to spot a drive that will tank, and what you can do about it.

How an SSD Writes Data

An SSD stores data in NAND flash memory cells, which are organized into pages (typically 4 KB) and blocks (often 4 MB or more). A critical limitation of NAND is that you can write to an empty page quickly, but you cannot overwrite a page that already has data. To reuse a page, the drive must first erase an entire block, and erasing is slow. To hide this, SSD controllers use a technique called write caching: they designate a portion of the NAND as a pseudo-SLC (single-level cell) cache. In SLC mode, each physical cell stores only 1 bit, so writes are fast and don’t require immediate erases. The controller writes incoming data to this cache first, then later moves it to the slower TLC or QLC (3-bit or 4-bit) area in the background. This is why a cheap SSD can show a burst of 500 MB/s for the first few gigabytes, then drop to 100 MB/s or less.

The SLC Cache: Size and Speed

The key spec that separates a good SSD from a cheap one is the size of the SLC cache and the speed of the NAND behind it. A typical budget 500 GB SSD might have a 5–10 GB SLC cache. If you copy a 20 GB video file, the first 8 GB goes at full speed, then the controller runs out of empty SLC pages. From that point, every new write must go directly to the TLC/QLC flash, and the drive also has to simultaneously erase and rewrite old cache data. The result is a write speed that can fall to 50–150 MB/s, which is slower than a mechanical hard drive in some cases. Higher-end drives use more overprovisioning (spare NAND), larger caches, and faster controllers to keep the cache from filling or to refill it quickly.

Why Cheap SSDs Are Worse

Cheap SSDs cut corners in three ways that directly affect large transfers:

• Small SLC cache: Budget drives often allocate only 5–10% of capacity to the cache, so it fills in seconds.
• Slow NAND: QLC (quad-level cell) NAND is cheaper but inherently slower to program than TLC; even the direct-write speed is lower.
• Weak controller: A low-end controller has fewer channels and less processing power, so it cannot manage background garbage collection efficiently while handling a heavy write stream.

Additionally, some cheap SSDs use a dynamic SLC cache that borrows space from the regular TLC area. This makes the cache appear larger, but it also means the drive has to do more background work to convert that borrowed space back, which can cause even steeper drops.

How to Test Your Own SSD

You can measure the drop yourself with a simple file copy test. Use a large file (e.g., 20–50 GB) and copy it from another drive or from an in-memory RAM disk. Use a stopwatch or a tool that shows transfer speed in real time. Watch the speed graph: you’ll see a high plateau, then a sudden cliff. The plateau length tells you the effective SLC cache size. To get a precise number, use a benchmark tool like CrystalDiskMark’s “All 0x00” fill test or a sequential write test with a large file size. But the manual copy test is enough to see the behavior.

How to Choose an SSD That Won’t Tank

If you regularly move large files, look for these specs:

• DRAM cache: A drive with a dedicated DRAM chip (usually 1 MB per 1 GB of capacity) maintains a mapping table that speeds up all operations, including cache management.
• TLC NAND over QLC: TLC is slower than SLC but much faster than QLC for sustained writes. Avoid QLC if you do heavy video editing or large backups.
• Large overprovisioning: Drives with more spare area (e.g., 10% or more) can maintain a larger effective cache.
• Published sustained write speed: Many manufacturers list a “sustained write” number; that’s the speed after the cache fills. Compare that, not the burst speed.

Common Misconceptions

One common myth is that the slowdown is due to the USB cable or the source drive. In reality, the bottleneck is almost always the destination SSD’s cache. Another misconception is that defragmenting an SSD helps; it doesn’t, and it wastes write endurance. Also, some people think that all SSDs are equally fast; the difference between a budget QLC drive and a premium TLC drive can be 5x for sustained writes. Finally, note that the slowdown is not a defect—it’s a design trade-off. The drive is doing background garbage collection, which is necessary to maintain performance over time.

What to Do If You Already Own a Cheap SSD

You can’t change the hardware, but you can work around the cache limit:

• Copy files in chunks: Pause between transfers to let the drive flush its cache. For example, copy 5 GB, wait 30 seconds, then copy the next 5 GB.
• Use a slower source: If you’re copying from a fast NVMe drive, the SSD gets hammered. Copying from a slower drive (like a hard disk) gives the SSD time to keep up.
• Enable TRIM: Make sure TRIM is enabled (default on Windows 10/11 and modern macOS) so the drive can clean up deleted blocks efficiently.
• Leave free space: Keep at least 15–20% of the drive empty. A nearly full drive has less room for the SLC cache and forces more frequent erases.

Frequently Asked Questions

Why does my SSD slow down after a few GB?

Because the SLC cache fills up. The first few gigabytes are written to a fast pseudo-SLC area; once it’s full, writes go directly to the slower TLC/QLC NAND, and the drive also starts background garbage collection, causing the speed drop.

Is it bad for the SSD to copy large files?

No, it’s normal. The drive is designed to handle sustained writes, but it will run hotter and use more write endurance. The slowdown is a protective measure, not a failure.

Can I increase the SLC cache size?

No, it’s fixed by the controller firmware. Some drives allow you to adjust overprovisioning, but that only slightly changes the cache behavior; you can’t make a QLC drive into a TLC drive.

Why is my cheap SSD slower than my hard drive for big transfers?

Because after the cache fills, the sustained write speed of a budget QLC SSD can be 50–100 MB/s, which is similar to or slower than a 7200 RPM hard drive. The hard drive doesn’t have a cache that fills; it just writes at a constant (though low) speed.

Does the DRAM cache matter for large transfers?

Yes, but not directly. DRAM helps with random access and mapping, but the SLC cache size is the main factor for sustained writes. A DRAM-less drive can still have a large SLC cache, but it may struggle with background tasks.

Should I buy a more expensive SSD just for large files?

If you regularly copy 20 GB+ files, yes. Look for a drive with TLC NAND, a large SLC cache (at least 10% of capacity), and a good sustained write rating. For casual use, a cheap SSD is fine.

Related

• How to Calculate Molarity in Chemistry
• How to Calculate Ocular Magnification
• 10 Minutes of Quality Time Jonah: A Guide