Mac Suddenly Slow on Local Network (SMB, NFS, SCP, SSH, Ping, etc. Are All Slow)

2025-06-21 ｜ Web ｜ #Words: 1132 ｜中文原版

Today, SMB suddenly got slow again—only about 8-9 MB/s. It’s well-known that Apple’s SMB is prone to sudden slowdowns. I tried installing NFS, but it was still slow. I was frustrated. I checked other scenarios like scp, and they were all slow too—yet my internet download speed was still fast.

For example, here’s the iperf test result:

iperf test result showing slow network speed

To put it bluntly: if only one service is slow, it’s a service issue. But when everything is slow, that’s scary.

During this time, I tried countless methods, read numerous posts and documents—but none worked (I’ll link some useful ones at the end; they didn’t help with this specific issue, but are good for expanding knowledge).

When I ran out of ideas, I started using Ping to check the route. I suspected something was interfering in the middle (I wasn’t even using a switch—just a direct connection), but unfortunately, there was nothing unusual. I also tried different Ethernet cables, a USB-to-RJ45 adapter, and the RJ45 port on my monitor. I even tested with Windows, and it worked normally.

After hours of frustration, I accidentally unplugged the Ethernet cable while pinging—and discovered something shocking: the IP packets were being sent via Wi-Fi but received via the Ethernet interface (below, Src is the IP of Mac’s built-in Ethernet interface, Dst is the IP of Ubuntu’s Wi-Fi interface):

Ping result showing Wi-Fi send and Ethernet receive mismatch

Wow, finally found the problem!

Unfortunately, I didn’t run netstat -rn to check before fixing it. But afterward, when DHCP was enabled again for automatic IP assignment, netstat -rn showed that the network interface (Netif = network interface) for accessing IP 169.254.2.7 was en1 (spoiler alert):

netstat -rn result showing en1 as the interface for target IP

And en1 is the BSD device name for Wi-Fi:

Network interface list showing en1 as Wi-Fi

Also, the Ping latency was way too high for a twisted-pair cable. It’s just a short cable—I even tried a 15cm one, but it didn’t help (look at the max/average/min values below; the fluctuation is huge):

Ping result with high latency and large fluctuation

You might ask: Didn’t you suspect Wi-Fi was the culprit when you noticed the slowness?
I did! I even checked the router admin interface, but unfortunately, it only showed internet speed—not LAN speed. I also forgot to check the TX/RX counters.

Now that we’ve identified the root cause, it’s time to fix it.

Why was I so sure it was an IP issue?
Three years ago, I made a video about the Dell U2723QE monitor (you can probably find my Bilibili account through it, haha). A comment said the monitor’s Ethernet port didn’t work.
When I tested it, I found that macOS’s DHCP is slow—sometimes it even fails to assign an IP, rendering the interface useless. The fix was to toggle the interface on and off.
Experience really does build up.

As you can see, the default gateway is 192.168.1.1 via en1. So traffic initially goes through Wi-Fi—after all, most modern Macs don’t have built-in Ethernet ports and rely on Wi-Fi.

The default subnet mask was 255.255.0.0. Logically, the two interfaces shouldn’t conflict, but they did. Let’s try manually setting the IP address and subnet mask. Here’s what I changed it to:

Manual IP and subnet mask configuration on macOS

Okay, let’s test again.

IP address and interface are correct:

Verified correct IP address and network interface

iperf test shows full Gigabit speed:

iperf test result showing full Gigabit speed

SMB server transfer speed is also correct:

SMB transfer speed showing normal performance

Perfect! It worked immediately after the change.

If you’re still having issues, check the server’s netmask (or Genmask). Try to minimize the broadcast domain. For example, when I ran netstat -rn on the server, I found both interfaces were mapped to the 169.254.x.x IP block—this could naturally cause the problem I faced:

Server netstat -rn result showing overlapping 169.254.x.x IP blocks

Updated on June 28, 2025
I explained why both interfaces map to the 169.254.x.x IP block in my article What’s the Difference Between Private-Use Networks and Link Local? What’s the Relationship Between Peer-to-Peer Wi-Fi and Wi-Fi Direct? If you’re interested, check it out.

I’m not sure how universal this fix is, but I’ll update this post if I encounter similar issues in the future.

I hope these will help someone in need~

Mac Suddenly Slow on Local Network (SMB, NFS, SCP, SSH, Ping, etc. Are All Slow)

Further Reading