Internet Speed

Don't assume if everything else is okay it is not your modem and router. Please unplug the modem and router from their power connection, then plug in the modem first and then the router and give them about10 minutes to fully come up to speed, then restart the computer in Safe Mode and renew the DHCP Leas on the Mac and then restart normally and test it. Please do everything in the order described. Also, verify your cables are in perfect shape.

It is likely a setting, you can start by resetting the lease in the slower iMac and if the connection is still slow please unplug the modem, unplug the router and then turn on each, then after they fully come up please restart the computer.

On a Macintosh computer:

1. Choose System Preferences from the Apple menu.
2. Choose Network from the View menu.
3. Select Wi-Fi, then click the Advanced button in the lower-left hand corner of the screen.
4. Select the TCP/IP tab from the top of the screen.
5. Click Renew DHCP Lease and click OK.

You didn't mention the type of cable in your LAN, but there is a table in Ethernet Cables Explained that shows speed caps by cable category and cable length. Any kinks in a cable that otherwise should support your expected bandwidth will reduce its speed. Your older iMac may be limited by its Wi-Fi chip capability and internal/external building interference.

I am still using a gen 4 Apple AirPort Base Station and the devices that see it (including two M1 Macs) via wireless usually max out at 300Mbps. The lone exception is the 2020 iMac 27-in Core I7 that reports this Wi-Fi connection at 405Mbps. None of my devices are more than 7 meters from the router. Waiting on the new Synology Wi-Fi 6 router to be released…