SSD speeds are sold as a single number: 7,000 MB/s, 14,000 MB/s, and on the box it looks like a ranking. In practice, that figure describes one idealised burst, and it predicts very little about how a drive behaves when you copy a 200 GB camera card, open a 50,000-image catalogue or load a game. A drive that writes at 2,000 MB/s for ten seconds can be slower than one that writes at 900 MB/s all day. This guide explains which numbers matter, where the bottleneck actually sits, and how to choose a drive for editing, photo work and gaming without paying for performance you cannot use.
By the end you should be able to read a specification sheet and predict how a drive will feel in your own workflow.
The short version
- SSD speed is more than the headline: random performance and sustained write decide real-world feel.
- SLC cache makes a drive look fast until the cache fills, then it falls to its true write rate.
- DRAM helps random writes and sustained traffic; HMB is a useful compromise.
- PCIe 5.0 is faster on paper, but games and app launches barely notice, and it runs hotter.
- Capacity, NAND type and free space matter as much as the interface.
The three SSD speed numbers on the box
Sequential read and write describe large blocks under ideal conditions: a single big file, a deep queue, a cool drive. Random performance measures small reads and writes scattered across the drive, usually quoted in IOPS at a given queue depth. Queue depth is how many requests are in flight; consumer workloads often run at QD1 to QD4, while benchmarks may use QD32 or more. A drive tuned for high queue depths can look spectacular in a benchmark and feel ordinary on a desktop.
Two benchmarks can disagree because they use different block sizes, queue depths and test durations. CrystalDiskMark’s sequential test is not the same as copying a folder of raw files. For a buying decision, look for sustained write and random read at low queue depth, not the biggest sequential number.
Where the bottleneck actually sits
Your SSD is one link in a chain: CPU, PCIe lanes, the drive’s controller, NAND, the operating system and, on a laptop, thermal limits. A PCIe 5.0 drive in a PCIe 4.0 slot runs at PCIe 4.0 speed. A fast internal drive behind a 10 Gbps USB enclosure is limited to about 1 GB/s. A DRAM-less QLC drive can slow to a crawl when its cache is exhausted. Upgrading the drive will not fix a CPU-bound export.
| Interface | Theoretical | Practical ceiling | Where you meet it |
|---|---|---|---|
| SATA III | 6 Gbps | ~550 MB/s | Older laptops and 2.5-inch drives |
| PCIe 3.0 x4 | ~3.9 GB/s | ~3.5 GB/s | Older laptops and desktops |
| PCIe 4.0 x4 | ~7.9 GB/s | ~7 GB/s | Current mainstream |
| PCIe 5.0 x4 | ~15.8 GB/s | ~14 GB/s | High-end desktops, needs cooling |
| USB 3.2 Gen 2 | 10 Gbps | ~1 GB/s | Portable enclosures |
| USB4 / Thunderbolt 3/4 | 40 Gbps | ~3 GB/s | Shared with display traffic |
| Thunderbolt 5 | 80 Gbps | ~6 GB/s | Newer hosts and enclosures |
SLC cache and the sustained-write cliff
TLC and QLC NAND store several bits per cell. Writing fewer bits per cell is faster, so drives use a portion of the NAND as SLC cache: incoming data lands quickly, then is folded into denser storage in the background. While the cache lasts, you see the headline speed. When it fills, write speed falls to the drive’s sustained rate, which depends on NAND, controller and free space.
| Drive class | Burst write | Sustained write (typical) | Best use |
|---|---|---|---|
| TLC with DRAM | 5–7 GB/s | 1–2 GB/s | Boot, editing, heavy writes |
| TLC DRAM-less | 3–5 GB/s | 500 MB/s–1.5 GB/s | General use, games |
| QLC with DRAM | 2–4 GB/s | 150–400 MB/s | Read-heavy library |
| QLC DRAM-less | 1–3 GB/s | 80–200 MB/s | Budget storage |
Those are illustrative ranges, not measurements. The cliff matters for video: a 4K ProRes 422 HQ clip at 707 Mbps writes about 88 MB/s, which almost any drive sustains. An 8K ProRes 422 HQ clip at 30p is roughly four times the data, around 350 MB/s, which is still within TLC territory but can expose a full QLC drive.
DRAM, HMB and DRAM-less drives
DRAM on the drive caches the flash translation layer, the map from logical addresses to physical NAND. Without it, the controller must fetch that map from NAND or use Host Memory Buffer, a small slice of system RAM. HMB helps, but DRAM-equipped drives usually handle random writes and sustained traffic better. For a boot or editing drive, DRAM is worth paying for; for a read-mostly game library, a good DRAM-less drive is fine.
Thermal throttling and the cost of a full drive
Fast drives produce heat, and sustained writes can push a controller into throttling. PCIe 5.0 drives often need a heatsink; laptops may not accommodate one. As a drive fills, the SLC cache shrinks and garbage collection has less room to work, so performance falls. Leaving 15–20% free is the simplest way to keep a drive responsive.
What real workloads demand
| Workload | What matters | Why |
|---|---|---|
| 4K H.265 editing | Sequential read, CPU decode | The drive is rarely the bottleneck |
| 4K ProRes editing | Sustained read, random access | Large intraframe files, easy playback |
| 8K RAW | Sustained write/read, thermals | High bitrate exposes cache and heat |
| Photo catalogue | Random read at QD1, capacity | Thumbnails and previews |
| Game loading | Random read at QD1–QD4 | PCIe 4 versus 5 is a small gap |
| Large file copy | Sustained write | The SLC cache cliff |
A 100 GB copy takes about 100 seconds at 1 GB/s, 33 seconds at 3 GB/s and 200 seconds at 500 MB/s. Those figures are arithmetic, not measurements. For a 4K H.265 edit, the CPU is usually the bottleneck; for 8K RAW, the drive and thermals matter. Photo catalogues stress small random reads and capacity more than sequential speed. Our camera codecs explainer covers the bitrates that sit behind these numbers.
What to actually do
Buy by capacity first, then TLC and DRAM, then interface. A 2 TB TLC drive with DRAM is a better editing purchase than a 1 TB PCIe 5.0 drive without it. For a game library, a good QLC or DRAM-less drive is acceptable. Check TBW and warranty, keep 15–20% free, and put the drive in an enclosure that can sustain its speed.
If you are buying a laptop rather than a drive, the same logic applies to the internal storage tier. Our MacBook Air M5 review explains why external storage is often the better upgrade, and the MacBook Air vs iPad Pro comparison covers the workflow differences. The computers section has more buying advice.
Common myths
“PCIe 5.0 makes games load faster.” Game loading is dominated by random reads at low queue depth and by CPU work. The difference between good PCIe 4.0 and PCIe 5.0 drives is small.
“Sequential speed is all that matters.” It predicts large copies, not app launches, catalogue browsing or sustained camera offloads.
“DRAM-less drives are always bad.” A good DRAM-less TLC drive is fine for read-heavy use. The weakness appears under sustained random writes.
“SSDs do not slow down when full.” They do: the SLC cache shrinks, garbage collection gets harder and write speed falls.
FAQ
Do you need a DRAM cache?
For a boot or editing drive, yes: DRAM improves random writes and sustained performance. For a secondary game or media library, a DRAM-less drive with HMB is usually adequate.
Is PCIe 5.0 worth it for video editing?
Only for very high-bitrate workflows and large sequential copies. Most 4K editing is CPU-bound, and a good PCIe 4.0 drive with DRAM is the better value.
Why does my SSD slow down after a few seconds?
Its SLC cache has filled. The drive is now writing at its sustained NAND speed, which can be several times slower, especially on QLC drives.
How much free space should I leave?
Around 15–20%. Free space gives the controller room for garbage collection and keeps the SLC cache from shrinking too far.
Is QLC good enough for a game drive?
Usually yes. Games are read-heavy after installation, and QLC’s slower sustained writes matter mainly when installing or updating large titles.
Bottom line
SSD speeds are a set of trade-offs, not a single ranking. Sequential numbers describe ideal bursts; random performance, sustained write, DRAM and thermals decide how a drive feels. Buy capacity first, prefer TLC with DRAM for editing, accept QLC or DRAM-less drives for read-heavy libraries, and check the interface your machine can actually use. A well-chosen PCIe 4.0 drive will usually beat a badly chosen PCIe 5.0 one in the work you do every day.
Evidence note: technical figures are drawn from published interface specifications and manufacturer data. Performance ranges are illustrative, not measurements; recommendations are editorial judgement.
