You bought a CFexpress 4.0 card rated past 3,000 MB/s, and the offload still crawls. The card is rarely the problem. The reader between the card and your machine almost always is.
The reader is the wall
A CFexpress 4.0 Type B card can read at around 3,500 MB/s. Put it in a USB 3.2 Gen 2 reader and you get about 850 to 900 MB/s, no matter what the card can do. That reader is a 10 Gbps pipe, and the card cannot push past it.
The upgrade that actually helps is a USB4 or Thunderbolt reader. In testing, the OWC Atlas USB4 reader moved data at over 3,300 MB/s, letting the same card read near its rating. For most shooters that one swap is the biggest real speed gain available.
The card is fine. The pipe is too narrow.
Read your own bottleneck
Watch the transfer rate during an offload and match it to the pipe:
- Around 850 to 1,000 MB/s. A 10 Gbps reader (USB 3.2 Gen 2). This is the common cap.
- Around 1,800 to 2,000 MB/s. A USB 3.2 Gen 2x2 reader.
- Near the card's rating. A USB4 or Thunderbolt reader.
If the number parks at 900 and stays there, the reader is your limit, not the card.
The drive on the other end counts too
A copy runs only as fast as its slowest link. A USB-C 10 Gbps portable SSD writes around 900 MB/s, so it drags a 3,500 MB/s card down to 900. Buy a destination drive for its sustained write, not its headline figure. Many consumer NVMe drives hold the rated speed for about ten seconds of cache, then fall to roughly 1,400 MB/s on a long copy.
The short version: if your offload sits well below the card's rating, check the reader first, then the destination drive. A faster card will not fix a 10 Gbps reader.
Sources
- Lexar, Why your CFexpress 4.0 card isn't reaching full speed
- Shutter Muse, OWC Atlas CFexpress 4 USB4 reader review
- Lexar, NVMe vs SATA SSD (sustained write and SLC cache)
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