Why your USB-C hub says 4K and still runs at 30Hz

A hub that advertises “4K HDMI” has told you almost nothing. 4K at 30Hz and 4K at 60Hz are the same number of pixels and two completely different experiences — one is usable for spreadsheets and unpleasant for anything that scrolls, the other is normal.

The reason is not marketing dishonesty, although it often looks like it. It is a bandwidth budget, and once you can do the arithmetic you can predict what a hub will actually do before you buy it.

The budget nobody prints on the box

USB-C carries video using DisplayPort Alt Mode, which borrows the high-speed lanes normally used for USB data. A USB-C connector has four of these lanes. How they are divided is the entire story.

Configuration	Lanes for video	Video bandwidth	What is left for USB data
4-lane DP Alt Mode	4	~25.9 Gbps (HBR3)	USB 2.0 only
2-lane DP Alt Mode + USB 3.2	2	~12.9 Gbps (HBR3)	USB 3.2 Gen 2 (10 Gbps)

Now the demand side. An uncompressed 4K frame at 60Hz with 8-bit colour and no chroma subsampling needs roughly 12.5 Gbps. At 30Hz it needs roughly half that.

Put those two tables together and the constraint falls out:

• 4K60 needs the full 4-lane budget. That leaves USB 2.0 for your peripherals — fine for a keyboard and a mouse, useless for an external SSD.
• If you want USB 3 speeds, video drops to 2 lanes. 12.9 Gbps is just enough for 4K60 if nothing else is competing, which is why some hubs manage it — and why the same hubs fall back to 4K30 the moment you plug a fast drive into them.

That is the whole trick. A hub that lists “4K HDMI + USB 3.0 + 100W PD” is describing three features that cannot all run at their maximum simultaneously, because they are drawing on the same lanes.

Where the brands actually differ

The three names you will hit first — Anker, UGREEN and CalDigit — sit at different points on this trade-off, and the difference is in what they document rather than in exotic engineering.

Anker (PowerExpand / 500 and 600 series). Broad retail availability, prices that make them the default choice, and spec sheets that are usually accurate but rarely state the lane allocation. If a listing says “4K@60Hz” and separately advertises USB 3.0 ports, the honest reading is that 4K60 works when the USB 3 ports are idle. Anker’s own product pages frequently footnote this; the retail listing usually does not.

UGREEN (Revodok series). Similar architecture, often a generation ahead on port count for the price, and marginally better at publishing a compatibility table. The same lane arithmetic applies. UGREEN’s advantage is documentation depth — they publish which laptops and which resolutions are supported, which is more useful than a port list.

CalDigit (TS3 Plus, TS4). The expensive end, and the price buys a different approach: Thunderbolt instead of plain USB-C. Thunderbolt carries video and PCIe data on separate allocations, so the 4K60-versus-USB-speed fight largely disappears. The catch is that you need a Thunderbolt host — a plain USB-C laptop gets USB-C behaviour from a Thunderbolt dock, not Thunderbolt behaviour.

The pattern: below roughly the CalDigit price point you are buying a lane-splitting hub and should assume the trade-off exists. Above it you are buying a different bus and the trade-off mostly goes away — if, and only if, your laptop actually has Thunderbolt.

HDMI version is the second trap

Even with a perfect 4-lane allocation, the hub’s HDMI output has its own ceiling:

HDMI version	Max at 4K	Typical appearance
HDMI 1.4	4K @ 30Hz	Most sub-$60 hubs
HDMI 2.0	4K @ 60Hz	Mid-range hubs, needs the full lane budget
HDMI 2.1	4K @ 120Hz	Rare in hubs, common on docks

A hub with HDMI 1.4 physically cannot do 4K60 no matter how much bandwidth you feed it. This is the single most common cause of “my hub says 4K but Windows only offers 30Hz” — the port is fine, the silicon behind it is not.

What to check on the listing, in order

1. Find the refresh rate next to the resolution. “4K@60Hz” is a claim. “4K” alone is not. If the listing only says 4K, assume 30Hz.
2. Look for a footnote about simultaneous use. Good listings say something like “4K@60Hz supported when USB 3.0 ports are not in use”. If that footnote exists, believe it — it is the honest version of the lane table above.
3. Check the HDMI version, not just the resolution. HDMI 2.0 is the minimum for 4K60.
4. Check the power delivery passthrough number against the input. A hub advertised as “100W PD” typically passes 85-90W to the laptop, because the hub itself draws power. If your laptop needs 90W and the hub passes 85W, you will slowly discharge under load.
5. Check whether your laptop’s USB-C port supports DP Alt Mode at all. Some budget laptops and most older ones do not. No hub fixes this.

How to verify it in five minutes after it arrives

You do not need to trust the box. Plug it in and check:

• Windows: Settings → System → Display → Advanced display. The refresh rate dropdown tells you the truth immediately. If 60Hz is not offered, the hub is running 4K30.
• macOS: System Settings → Displays, hold Option while clicking “Scaled” to see the full list of modes.
• Then load it. Start a file copy to an external drive while watching whether the refresh rate drops. If it does, you have reproduced the lane split and you now know which mode your hub prefers.
• Check the power draw. A laptop that reports “plugged in, not charging” under load is a hub passing less than the laptop wants.

Who this matters to, and who can ignore it

This matters if you run a 4K monitor at 60Hz as your primary display, or you want a fast external SSD and 4K60 at the same time. In both cases the cheap hub will disappoint you in a way that is hard to diagnose after the fact.

You can ignore all of it if you are on 1080p or 1440p. Those resolutions fit comfortably inside a 2-lane budget at high refresh rates, which is why a $30 hub works perfectly for most people and the entire problem is invisible to them.

Skip the hub entirely if your laptop has a dedicated HDMI or DisplayPort output. Routing video through a hub for no reason adds a failure point and a bandwidth constraint you did not need.

The honest summary

A USB-C hub is not a switchboard, it is a budget. Every port on it is spending from the same allowance, and the manufacturer decides how to divide it — sometimes in a way that makes the box look better than the product.

If you need 4K60 plus fast storage plus charging, the reliable answer is a Thunderbolt dock and a Thunderbolt host. If you need 4K60 and can live with USB 2.0 peripherals, a well-documented USB-C hub at half the price will do it. What will not work is assuming that a port list is a capability list.