SLC caching: why SSD write speed can drop during long copies
SLC caching uses a portion of NAND flash in single-level cell mode to accelerate writes. Once the cache fills, the drive writes directly to TLC or QLC media, which is slower. The drop in speed during long copies is normal and depends on the drive's design and workload.
What is SLC caching
Solid state drives store data in NAND flash memory, which is organized into pages for writing and blocks for erasing. Most consumer SSDs use TLC or QLC NAND, storing three or four bits per cell respectively, to increase capacity and lower cost. However, writing more bits per cell reduces performance and endurance, as noted by SNIA's SSD overview.
To improve write performance, many drives set aside a portion of the NAND to operate in single-level cell (SLC) mode, where each cell stores only one bit. This SLC cache accepts incoming writes at high speed, acting as a fast buffer. The drive later moves the data to the denser TLC or QLC area in the background, a process that can happen when the drive is idle.
Why write speed drops during long copies
The SLC cache has a finite size, typically a few gigabytes or tens of gigabytes depending on the drive. When a copy operation writes more data than the cache can hold, the cache fills up. At that point, the drive must write directly to the TLC or QLC media, which is slower because each cell holds more bits and requires more precise voltage levels, as explained by Solidigm's QLC workload brief.
The sustained write speed after the cache fills is often much lower than the burst speed. This is not a defect but a design trade-off. Consumer workloads are typically bursty, such as moving a few large files or installing an application, and leave the drive idle most of the time, allowing the cache to recover, as described in SNIA's SSD education page.
How SLC caching affects endurance
SLC caching also influences drive endurance. Writing in SLC mode causes more physical writes to the NAND because data is written once to the cache and then again to the final location. This write amplification can reduce the drive's lifespan, but manufacturers mitigate it with overprovisioning and firmware optimizations, according to Samsung's SSD FAQ.
Endurance is specified in terabytes written (TBW) or drive writes per day (DWPD), and it varies with workload. Random writes and small block sizes cause more write amplification than sequential writes, so the actual endurance can differ from the rated value, as noted by SNIA's endurance discussion.
Checking a drive's SLC cache behavior
To understand a specific drive's SLC cache size and sustained write speed, users can consult the manufacturer's specifications or independent reviews that measure sustained write performance. Benchmarking tools can show the drop in write speed after the cache fills, but results vary with the tool and system configuration, as stated in Samsung's performance check FAQ.
For users who frequently copy very large files, such as video editing laptops, a drive with a larger SLC cache or higher sustained write speed may be preferable. Enterprise drives often have different caching strategies and higher endurance to handle continuous writes, as explained in Solidigm's PCIe Gen 4 workload guide.
NAND flash technology itself influences the caching strategy; for example, Micron's product page notes that SLC NAND offers high performance and write endurance, while QLC NAND provides greater density at lower cost, which affects how manufacturers design SLC caches (Micron's NAND flash page).
| Cell type | Bits per cell | Relative write speed | Relative endurance | Common use |
|---|---|---|---|---|
| SLC | 1 | Highest | Highest | Caching, enterprise |
| MLC | 2 | High | High | Older consumer, enterprise |
| TLC | 3 | Moderate | Moderate | Mainstream consumer SSDs |
| QLC | 4 | Lower | Lower | High-capacity consumer, read-intensive |
Frequently asked questions
Why does an SSD slow down after copying a large file?
The SLC cache fills up, and the drive must write directly to the slower TLC or QLC NAND. This is normal and depends on the cache size and the drive's design.
Is the write speed drop a sign of a faulty SSD?
No, it is a characteristic of SLC caching. The drive is designed to provide high burst performance, but sustained writes are limited by the underlying NAND type.
Can a user increase the SLC cache size?
Some drives allow adjusting overprovisioning, which can indirectly affect cache size, but the SLC cache is typically fixed by firmware. Check the manufacturer's software for options.
Does SLC caching wear out the SSD faster?
It can increase write amplification, but manufacturers use overprovisioning and wear leveling to mitigate this. The endurance rating (TBW) accounts for typical usage.
How can a user find the sustained write speed of an SSD?
Look for independent reviews that measure sustained write performance, or use benchmarking tools. The manufacturer's specifications may list sequential write speeds, but sustained speeds are often not published.
