Thunderbolt May Not Be As Fast As You Think

[ Updated Oct. 15, 2023, with a link for Thunderbolt 5. ]

Thunderbolt is an amazingly fast protocol for connecting computers to storage devices. However, even though it has potentially blazing speed, if you don’t understand how it works, you may not be getting the speed you expect.

A MATTER OF PLUMBING

An effective way to think about moving data between storage and a computer is to think about plumbing. Yup. Pipes and water pressure.

• If you have a very small pipe, such as a garden hose, it can only fill a swimming pool very slowly even with a lot of water pressure behind it.
• If you have a very large pipe, such as one of those things buried under a street, it can fill a swimming pool very, very quickly; provided you have a lot of water pressure behind it.
• However, if the water pressure is low, even a very large pipe won’t carry a lot of water.

Thunderbolt defines the size of the pipe, but it doesn’t determine water pressure. The type of storage hardware and how it is configured determines the water pressure.

NOTE: Um, no. Data is not water, this is an analogy. Your computer is not getting wet.

THUNDERBOLT – The Size of the Pipe

Thunderbolt is the name of a hardware interface, co-developed by Intel and Apple, that connects external devices to a computer. It was first released in 2011. It combines two signals into one cable (or “pipe”): PCIe, for data, and DisplayPort, for monitors. It also has the ability to carry power; though the wattage varies.

Thunderbolt is optimized for rapid transfer of large files, which is why it is so useful for video editing. There are four versions of Thunderbolt, with a fifth version rumored but not announced:

• Thunderbolt 1
• Thunderbolt 2
• Thunderbolt 3
• Thunderbolt 4

The principle difference between versions 1, 2 and 3 is the size of the pipe they create. (The bandwidth of Thunderbolt 3 and 4 is identical.) However, that pipe is split into two parts: one for data and the other for monitor video.

Illustration of Available Data Rates

NOTES:

• Video signals take priority over data.
• The bandwidth of Thunderbolt 3 and 4 are identical.
• Data is always encoded during transfer. Decoding lowers the data bandwidth.
• Even when Thunderbolt carries no video signals, data bandwidth can not use the full bandwidth of a Thunderbolt pipe.

The amount of bandwidth required by video is dependent upon the monitor size, bit depth, refresh rate, and the number of monitors connected. It is entirely possible to fill almost all of a Thunderbolt pipe feeding monitors, leaving almost no bandwidth for data.

Monitor Bandwidth Requirements

NOTES:

• Video signals take priority over data.
• These numbers assume 10-bit video at 60Hz running on Thunderbolt 3.
• Video bandwidth is subtracted from the total Thunderbolt bandwidth to determine the maximum data bandwidth available.
• Source: MacPerformanceGuide.com

So, as an example, if you are running a 4K monitor at 60 Hz over Thunderbolt 3, the maximum available data bandwidth would be 40 – 15 = 25 Gbps; or roughly 3 GB/s. After decoding this data rate drops to about 2.6 GB/s. Running two 4K monitors over the same Thunderbolt 3 or 4 cable would reduce the maximum data bandwidth to roughly 10 Gbps, or roughly 1 GB/second.

Photo courtesy of Azamat Esenaliev, Pexels.com.

BUT WAIT, THERE’S MORE!

To quickly summarize: a large chunk of Thunderbolt bandwidth is devoted to video and is inaccessible for data transfer.

However, Thunderbolt only defines the size of the pipe. Your actual storage devices determine the maximum amount of data (the “water pressure”) that can flow through the pipe each second. Here’s a table that illustrates some of these differences.

Rough Data Transfer Rates for Typical Storage

NOTES:

• Even the fastest HDD RAIDs won’t fill a Thunderbolt 3/4 pipe.
• All RAID 1 configurations, regardless of the number of devices, have the speed of a single device.
• NVMe SSDs, when combined as a RAID, exceed the data bandwidth provided by Thunderbolt. The reason for using multiple NVMe devices in a Thunderbolt RAID is to increase storage capacity.

Another weirdness of storage is that transfering large files is much faster than transferring smaller files. For example, transferring a 10 GB file will come close to matching these maximum speeds. Transferring, say, fifty 1 MB files to an HDD will average about 15 MB/sec.

SUMMARY

Now that our brains have exploded, here are the key takeaways:

• Video signals take priority – and bandwidth – in a Thunderbolt pipe
• Even without carrying video, data can only access a portion of Thunderbolt bandwidth
• The storage you connect to Thunderbolt determines the maximum speed of data transfer available.
• Transferring large files is much faster than transferring smaller files.

Thunderbolt is an amazing protocol and essential to many video workflows. However, it isn’t a magic box. Once you understand what affects the speeds it can transfer data, you can better plan for how to use it.

EXTRA CREDIT

In September, 2023, Intel introduced a new version of Thunderbolt, dubbed “Thunderbolt 5,” that promises an 80 Gbps bandwidth! Here’s a link to learn more.