Why a 240W USB-C Cable Can Be Slow—and What to Buy for an SSD or Monitor
The same USB-C cable charges your laptop at full speed, yet your external SSD copies at a crawl and the monitor stays black. What is the 240W rating not telling you?
A cable can charge a laptop successfully and still make an external SSD feel slow. It can also deliver power from a monitor without displaying an image. The USB-C plug fits all these connections, but the cable's supported functions determine what travels through it.
This combination appears in ordinary manufacturer specifications. ASUS lists its ROG 240W USB-C cable, model LCR50, with 240W power support and a 480Mbps data rate. Buying it for high-power charging does not also buy a high-speed SSD connection. Both numbers are present on the ASUS specification page.
Read the power and data ratings separately
Watts describe the cable's supported power capacity. A 240W cable does not force 240W into a phone, nor does it make a lower-powered charger deliver more energy. The charger and receiving device must support the charging mode and power level being used.
Mbps and Gbps describe data rates. A 480Mbps connection is 0.48Gbps, far below a 10Gbps connection. Power delivery and data transfer have different requirements, so a cable can combine high-power charging with USB 2.0 data. That is a valid specification combination, even when it is a poor match for your intended use.
USB-IF's current certification program requires USB-C-to-USB-C cables to carry a 60W or 240W power marking. It also requires speed markings except for USB 2.0 cables. A power number by itself therefore does not establish a fast data connection. These are certification marking rules, rather than a requirement that every cable already in circulation display the newest logo.
What 480Mbps means for a large file
The lowercase “b” in Mbps means bits; the uppercase “B” in file sizes means bytes. Eight bits make one byte. Dividing 480Mbps by eight gives 60MB/s, while 10Gbps becomes 1,250MB/s. Comparing an SSD's MB/s rating directly with a cable's Mbps rating skips that conversion.
| Advertised link rate | Converted bandwidth | Arithmetic for 10GB |
|---|---|---|
| 480Mbps | 60MB/s | About 167 seconds |
| 10Gbps | 1,250MB/s | 8 seconds |
Those times use decimal units: 10GB equals 10,000MB. They divide file size by raw bandwidth and exclude protocol overhead and storage delays. They are not measured copy times. Their purpose is to show why a USB 2.0 connection can hold back a drive advertised at hundreds or thousands of megabytes per second.
The cable also cannot upgrade either endpoint. A USB 3.2 Gen 2 SSD connected through Apple's Thunderbolt 4 Pro Cable still uses that cable's USB connection, rated up to 10Gbps. The 40Gbps Thunderbolt number does not turn a USB SSD into a Thunderbolt drive. The computer port, cable, and drive enclosure need a shared interface.
To investigate an existing slow setup, use the same computer port, drive, and file while changing the cable. Connect directly instead of through a hub for the comparison. A folder containing many small files introduces different work from a single large file, so changing the workload at the same time makes the result harder to interpret.
Real cables demonstrate the tradeoffs
The following manufacturer specifications were checked on October 26, 2026. They describe supported capabilities, with compatible equipment at both ends.
| Cable | Power | Data | Video |
|---|---|---|---|
| ASUS ROG 240W USB-C, LCR50 | Up to 240W | 480Mbps | Not specified on the cited specification page |
| Apple Thunderbolt 4 Pro Cable | Up to 100W | Up to 40Gbps over Thunderbolt or USB4; up to 10Gbps over USB 3.2 Gen 2 | Supported |
| Apple Thunderbolt 5 Pro Cable | Up to 240W | Thunderbolt 5 up to 80Gbps bidirectional, or 120/40Gbps with video boost; USB 3.2 Gen 2 up to 10Gbps | Supported |
The 100W Apple cable supports much faster data connections than the 240W ASUS cable. Conversely, someone who only needs charging may have no use for the extra data capabilities. Choosing by the largest wattage number would obscure both outcomes.
Apple also explicitly describes its ordinary USB-C Charge Cable as limited to 480Mbps with no video support. Its Thunderbolt Pro cables support video. This explains why substituting a charging cable for a monitor cable can break an otherwise working desk setup. Apple's cable comparison covers those functions separately.
A monitor needs a complete video connection
DisplayPort Alt Mode is one way to carry display signals through USB-C. The computer must provide that output, the cable must carry it, and the monitor or adapter must accept it. Charging can work even when this video path is incomplete. VESA explains the requirements in its DisplayPort over USB-C FAQ.
Start with the computer's port specifications. Look for DisplayPort Alt Mode or Thunderbolt support appropriate for the display connection. Then identify the monitor's USB-C video input. A monitor can have other USB ports intended for peripherals; plugging into a matching connector does not establish that it is the laptop input.
For an HDMI display, the adapter or cable must take video from the computer's USB-C output to the display's HDMI input. Match the desired resolution and refresh rate across that connection. A listing that says only “4K” does not tell you whether it supports the 4K refresh rate you want.
A single-cable desk adds further requirements. The monitor may also provide laptop power and relay a keyboard and mouse through its USB hub. That requires video, data, and sufficient charging capability together. Increasing the cable's power rating cannot increase the monitor's available power output.
Length belongs in the purchase specification too. Someone placing a monitor far from a laptop needs the capabilities at that particular cable length. Do not assume that every length sold under a product-family name has identical performance. A long charging cable is not automatically a replacement for the shorter video cable supplied with a display.
Why Thunderbolt 5 says 120Gbps
Thunderbolt 5 provides up to 80Gbps of bidirectional bandwidth. With substantial video traffic, Bandwidth Boost can allocate up to 120Gbps in one direction and 40Gbps in the other. Intel describes that boost as a way to accommodate demanding display traffic.
That does not promise 120Gbps SSD transfers or 15GB/s file copies. Storage performance also depends on the interface and storage devices involved. A USB 10Gbps drive can be served by a cable supporting that connection; buying a Thunderbolt 5 cable does not remove the drive's own limit. The additional capabilities matter when your computer and peripherals can use them.
Choose for the connection you actually use
For charging, match the power capability required by the device and charger, then choose an appropriate length. If this cable stays beside a charger and never connects storage or a display, faster data support may add no practical benefit.
For an SSD, identify the computer port's interface and the drive's interface, then buy a cable explicitly supporting their shared data rate. “240W fast charging” alone is insufficient information. If changing the cable does not improve the connection, compare a direct connection with the hub or dock path. A change only when bypassing the hub directs attention to that intermediate device.
For a monitor, write down the resolution, refresh rate, laptop charging requirement, and any peripherals attached to the monitor. Select a video-capable cable that supports that combination at the length you need. If the screen remains blank, select the actual connected input on the monitor and compare against a cable already known to carry video on the same ports.
If a shopping page lists only wattage, connector shape, and the braided finish, it has not answered the SSD or monitor question. Find the specification for the exact model and length, including the data rate and video support. A product photo of a USB-C connector cannot supply those missing capabilities. For a replacement cable, record the working cable’s model before ordering; that gives you an actual specification to match instead of relying on how similar the two cables look.
Once a connection works, label the cable by role: charging, SSD, or monitor. Keeping a display's supplied cable with that display prevents a later desk rearrangement from silently replacing it with a charging cable that happens to fit.