iMac Wi-Fi speed decreased after Golden Gate download

A drop from 540 Mbps to 450 Mbps after installing macOS 27 does not establish that macOS 27 caused the reduction. Internet speed can be affected by the iMac's Wi-Fi connection, router, ISP, speed-test server, background activity, VPN, or software that filters or reroutes network traffic.

First restart the iMac and repeat the test using the same speed-test service and, if possible, the same server. Keep the iMac in the same location and make sure there is no significant network activity.

Then check the iMac's Wi-Fi connection. Hold the Option (⌥) key while clicking the Wi-Fi icon in the menu bar. 

In the Wi-Fi details, note the Transmit Rate (Tx Rate), measured in Mbps, as well as the channel/band and, if shown, RSSI. 

The Tx Rate is the negotiated Wi-Fi link rate; it is not the same as your Internet speed-test result. A significant change in Tx Rate between tests can indicate that the wireless connection itself has changed.

Next disconnect any commercial VPN and temporarily disable any third-party security software or two-way firewall/network filter that can inspect network traffic. 

Test again after each change rather than changing everything at once. Apple identifies VPN and other software that monitors or interacts with network connections as possible sources of network problems. Apple: If your Mac isn't connecting to the internet over Wi-Fi

If the speed remains around 450 Mbps, test in Safe Mode and then from a secondary user account. 

Safe Mode helps determine whether software loaded during normal startup is contributing to the problem. 

A secondary account helps determine whether the problem is specific to your normal user environment.

If the speed returns toward 540 Mbps in Safe Mode or the secondary account, investigate software or network services active in the normal environment, particularly the VPN, security software, firewall, proxy, or network extensions. 

If it remains around 450 Mbps in all tests, compare another device on the same Wi-Fi network. 

If the other device also shows a similar reduction, the cause is less likely to be specific to the iMac.

If the problem remains, use Wireless Diagnostics and check Apple's recommended Wi-Fi settings.

If necessary, test with a separate network location before considering more extensive network configuration changes. Apple: Recommended settings for Wi-Fi routers and access points · Apple: Use Wireless Diagnostics · Apple: Change network settings using network locations

The key is to record the Tx Rate and speed-test result before and after each diagnostic change. This helps distinguish a change in the iMac's Wi-Fi connection from a change in Internet throughput.

One technical clarification: Tx Rate is not expected to equal your Internet speed.

For example, a Wi-Fi Tx Rate substantially above 450–540 Mbps can coexist with an Internet speed test in that range. What matters diagnostically is whether the Tx Rate itself changes significantly when the Internet speed changes.

Hi,

You can fix Wi-Fi slowdowns after updating to macOS Golden Gate by checking background processes, resetting network interfaces, or managing AWDL (Apple Wireless Direct Link) features.

Thanks for your reply, i am afraid I am unable to carry out your suggestions as I have no idea how to do it.

I tried contacting Apple Help but the tech was not very helpful.Which is why I posted here.

Who are you replying to, kaz-k or Owl-53. When you reply to someone reply to their post, not your own.