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I have a 2021 16" MacBook Pro and my wife has a 2019 15" MacBook Pro, both running macOS 12.3.1. I have a UniFi gigabit network with a Dream Machine Pro and Switch Lite 16, also running the latest firmware. I ran iperf3 with my hardwired PC as the server and each MacBook Pro as clients one at a time using the same Ethernet cable and Cable Matters USB-C to Ethernet adapter. Here are the results:

2019 MacBook Pro 2021 MacBook Pro

  • 2019 MacBook Pro: 911 Mbits/sec
  • 2021 MacBook Pro: 519 Mbits/sec

I also got the same results with:

  • A spare Cable Matters USB-C to Ethernet adapter using the same Ethernet cable
  • A Uni USB-C to Ethernet adapter using the same Ethernet cable
  • Each of the three adapters with another Ethernet cable
  • Each of the three adapters with another Ethernet cable on a different port on my switch
  • Repeating the above steps with all three USB-C ports on my 2021 MacBook Pro I could be wrong, but these results lead me to believe that it's not an issue with the network, adapter, or Ethernet cables, and that the issue is with my 2021 MacBook Pro.

Any ideas why my 2021 MacBook Pro is getting slower results than my wife's 2019 MacBook Pro?

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  • Are you on battery or AC power for these tests?
    – pion
    Commented Apr 10, 2022 at 3:53
  • @pion Just posted the solution - thanks.
    – Ryan
    Commented Apr 18, 2022 at 16:40

1 Answer 1

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Solved.

The issue is that macOS does not have the gigabit USB Ethernet drivers to support the Realtek RTL8153 chipset that is used in the USB-C to Ethernet adapters that most brands use (and that I tried).

The solution is to purchase a USB-C to Ethernet adapter with a Realtek RTL8156 chipset because macOS does have the drivers to support these adapters. These are usually 2.5Gbps adapters (not 1Gbps).

I purchased the Plugable 2.5Gbps adapter - https://plugable.com/products/usbc-e2500

Hallelujah gigabit speeds via iPerf3!

enter image description here

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  • Hi @Ryan - I've just come across this post and am actually having a similar issue using the same Pluggable USBC-E2500 (and other 2.5Gbps adapters). I get the full 1.2Gbps (the speed of my network) for a while, but then it seems to fall back to around 100Mbps. Any thoughts?
    – Eric
    Commented Jan 11 at 0:11

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