DirectConnectTransfers are slower than expected
First check the speed DirectConnect measured for your cable. If copies are much slower, the cause is usually how you’re copying or the disk at the other end.
Check the measured speed
The other computer’s window shows the connection type and the measured speed, for example “Connected via USB4 · 9.3 Gb/s”. That’s the most a copy can reach on this connection.
Expect it to be lower than the cable’s rating. For example, two Macs on a 40 Gbps Thunderbolt cable measured 22.2–23.5 Gb/s.
Copying in Finder or File Explorer
If you’re dragging files in Finder or File Explorer, use fast copy instead.
Finder and File Explorer copy one file at a time. For each file, they wait for the other computer to answer about a dozen requests: look up the name, create the file, write it, set its dates and close it. With thousands of small files, the waiting takes much longer than moving the data.
Fast copy copies up to 64 files at once and still checks every file. Between two Macs, Finder copied 2,000 small files in 2.3–2.8 seconds and fast copy took 0.23–0.71 seconds. Between a Windows PC and a Mac, File Explorer took 4.8–5.6 seconds and fast copy took 0.77–1.11 seconds.
Large files sent from a Mac are slow
Finder sends a large file to another Mac at about 6.6 Gb/s, even on a faster cable. macOS sends file data to network drives in small pieces, and DirectConnect can’t change that. Fast copy sent the same 1 GiB file at 11.3–13.8 Gb/s.
Copying from the other Mac in Finder isn’t affected: about 11.3 Gb/s.
The measured speed is low
- Use a faster cable. If both computers support 40 Gbps, a 40 Gbps Thunderbolt 4 or USB4 cable can be much faster than a 20 Gbps one. See which USB-C cable you need.
- Use the right port. Some computers have one fast port and slower others. Check your computer’s specifications.
- Plug in directly. Hubs and docks can slow the connection.
- On Linux, some laptops use only one of the cable’s two lanes. See Known issues on Linux.
Small files from a Windows PC are slow the first time
The first copy of a folder of many small files from a Windows PC can be more than ten times slower than the next, because Microsoft Defender scans each file the first time it’s opened. See Known issues on Windows.
Small files are slow while a large copy runs
Small files share the connection with a large copy between the same two computers, and wait behind its data.
For example, a Mac copied 2,000 small files to a Windows PC in Finder in 2.7–2.9 seconds. While the PC sent 1 GiB files to the Mac, the same copy took 17.8–21.5 seconds.
If you need the small files first, copy them before the large copy starts, or after it finishes.
Large copies slow down partway through
A copy much larger than the receiving computer’s free memory slows to the speed of its disk once the memory fills. An external drive or an older internal drive can be slower than the cable.
For example, a Mac copied a 40 GB file to a Windows PC at 5.9–6.0 Gb/s. The PC’s own disk wrote the same amount at 7.95 Gb/s with nothing else running, so the disk, not the cable, set the pace.