Quick Answer: A card reader with a built-in hub shares one USB link: 5 Gbit/s of signalling on USB 3.2 Gen 1, about 400 MB/s in practice, divided between the card slot and every downstream port. Bus power is 900 mA at 5 V for USB 3.x, 4,5 W in total, and a UHS-II card alone can pull 1,5 W while writing. Put the reader on its own host port if cards above 104 MB/s matter, and never chain it behind two hubs.
Two devices in one shell look like a saving until a card copy stalls because a keyboard and a phone are sharing the same 5 Gbit/s and the same 900 mA. Decide which of the two functions is critical, then check whether the shared link can carry it.
Where the bandwidth actually goes
Card speed against link speed
A UHS-I card tops out at 104 MB/s and a UHS-II card at 312 MB/s, while USB 3.2 Gen 1 moves about 400 MB/s after protocol overhead. One fast card nearly fills the link on its own, so a hub port carrying a second reader or an external drive halves the throughput of both.
Hub tiers and latency
USB allows five tiers of hub below the host, seven including the root. Each tier adds 0,5 to 1 ms of latency and shares the upstream bandwidth, so a reader behind two hubs on a docking station is the common cause of a card that mounts slowly and unmounts during a copy.
What each device can draw
| Device | Interface | Sustained rate | Bus current |
|---|
| UHS-II SD card | SD 4.0 | 312 MB/s | 1,5 W peak |
| UHS-I SD card | SD 3.0 | 104 MB/s | 0,5 W |
| CFexpress Type B | PCIe 3.0 x2 | 1,700 MB/s | 3 W |
| USB 3.2 Gen 1 port | 5 Gbit/s | 400 MB/s | 900 mA |
| USB 2.0 port | 480 Mbit/s | 40 MB/s | 500 mA |
Power budget and the dropout that follows
A reader plus a 2,5 inch drive plus a phone can exceed 900 mA on one hub port, and the symptom is not a clean error: the card disappears mid-copy and reappears, and the file system reports a dirty volume. Feed the hub from its own 5 V 2 A supply, or move the drive to a port on the host. A CFexpress reader writing at 3 W will brown out a bus-powered hub every time.
Choosing between one shell and two
A reader with a hub suits a laptop with two free ports and light cards: SD at 104 MB/s, keyboard, mouse. Two separate devices suit anything above that. The cost difference is small compared with a corrupted card, and a dedicated reader at 312 MB/s keeps its full rate. Where the hub is the main purchase, read usb hub ratings for upstream and downstream current before assuming the ports are independent.
Connector and cable limits
A USB-C reader with a captive 0,3 m lead is more reliable than a reader on a 1 m usb cable with a micro-B socket, which is the first part to wear. Check that the cable is rated for 5 Gbit/s; a 480 Mbit/s charge lead caps the reader at 40 MB/s and the card looks faulty. A type c hub with Power Delivery pass-through is the safer way to add ports to a single-port laptop.
Practical scenarios
- Situation: a photographer copies 64 GB cards at 312 MB/s through a bus-powered hub with a mouse and a phone attached. Move the reader to the host port and the copy time falls from 9 minutes to 3,5.
- Situation: a reception desk needs SD, a keyboard and a label printer. One reader with a three-port hub is enough; total draw is 380 mA and the shared 5 Gbit/s is never saturated.
- Situation: a laptop has one USB-C port and needs a card reader plus mains charging. Use a hub with PD pass-through at 60 W, and accept that the reader will run at 104 MB/s while charging.
- Situation: a CFexpress card unmounts during a 20 minute offload. Fit a powered hub at 5 V 2 A and the dropout stops, because the reader was pulling 3 W from a 900 mA budget.
FAQ
Does a hub slow down a card reader?
It shares one upstream link, so yes when other devices are busy. Alone on a 5 Gbit/s hub a UHS-II reader still reaches about 300 MB/s. With a drive copying at the same time, expect half of that.
Why does my card unmount during a copy?
Almost always current. A bus-powered hub gives 900 mA total, and a fast reader or a portable drive exceeds it in bursts. Fit a powered hub or move the reader to a host port and the fault disappears.
Is a hub with a card reader as fast as a separate reader?
Only for UHS-I cards at 104 MB/s. Above that the shared link and the shared current become the limit, and a separate reader on its own port is measurably quicker on large files.
