New 1Gbps fiber wifi not reaching speed

Wi- Fi Radio Signals from your Router fall off as the cube of the distance. Provided you are close to your Router and have clear line of sight and no competition from neighbors' Routers or known interference producers like microwave ovens.

All others need to do a more careful analysis to find out what is happening.

Wireless diagnostics:

Hold down the Option key while you click on the Wi-Fi icon on the menubar to open up the tools for investigating and fixing Wi-Fi issues

First is "Open Wireless diagnostics", which opens the wireless diagnostics Assistant, but does not proceed.

There are about eight different things you can do from here, but the top-level is to choose Diagnostics off the Window menu, or simply click continue, to do a quick check for Gross misconfiguration or operating problems. Your Admin password will be required. If any recommendations are shown, you should consider then seriously.

Next is to hold down the Option key and click the Wi-Fi icon as before. The screen that opens shows operating parameters of your Wi-Fi network. Screenshot, transcribe, or photograph the results and post back in a reply on the forums. Looks like this older one.

Do you have a separate modem and router? If so how is the router connected to the modem? If by ethernet cable try replacing the cable with the newer Cat 5e, 6a or 7 cable. I had a similar problem in that I was only getting 100 MBs from a 500 MBs fiber cable. It was the ethernet cable. I had an older, slower cable. I changed it and all devices are now at the 500 MBs speed.

Well, first question is are these devices all connected via wired ethernet or WiFi. Your connection and your router may be 1 gig capable but Wifi cuts down speed dramatically depending on location, distance, and local interference. Also check what version of WiFi your router supports, WiFi 5, 6, 6e, or 7 and what Wifi speeds your devices support. In my own personal experience my M1 Studio Max gets symmetrical ~960 down and up using wired ethernet while my iPhone 16 Pro, Apple TV, and iPad get half that using WiFi 6. Wifi routers make unrealistic claims about speed. Wireless connections are greatly influence by their environment.

Bottom line, 400-620 WiFi is about normal

Ethernet will be limited to just under 100 M bits/sec when the cable does not have all four pairs of wires present, or some wires are broken or mis-wired.

This is because up to 100 M bits/sec speeds, the signaling is baseband -- one bit per signaling interval (which only requires Two pairs of wires).

The next increment is 1000 M bits/sec (Gigabit Ethernet), which uses a modulated signal (multiple bits per signaling interval) and requires the full spectrum that can only be provided by four pairs of wires, properly Terminated.

An interesting aside [to me anyway] is that the Mac Studio does not support baseband 100 M bits/sec at all, and has a minimum of speed 1000 M bits/sec speeds, a modulated signal.

EDIT: I just checked the specs on current Mini models, and they ALSO have the restriction that speeds below 1000 M bits/sec are NOT supported.

If you are getting half or more than the claimed speed via wifi you are doing well.

Follow the advice you have been given.