Apple Extreme/Time Capsule with newly installed Full Fibre - slow network speed

To simplify things.......please temporarily, power off the AirPort Extremes and connect your Mac to one of the Ethernet ports on the back of the Time Capsule.

Let's first find out whether your Time Capsule is set up as a router for the network, or whether it is set up as a "bridge".

Open AirPort Utility on your Mac

Click on the picture of the Time Capsule

Click Edit in the smaller window that appears

Click the Network tab at the top of the next window

What is the setting for Router Mode?

It will either be DHCP and NAT.....or......Off (Bridge Mode)

If the Time Capsule is set up as a DHCP and NAT router, the top speed will be in the 300-325 Mpbs range and might well be lower.......since the Time Capsule was never designed to work with an ONT.

It was instead designed to work with a standard DSL or cable modem.

If the Time Capsule Router Mode is Off (Bridge Mode), the Time Capsule is not acting as the network router and it might be able to achieve speeds in the 700-800+ Mbps range.

Post back on your progress and we will continue.

I am assuming that your Time Capsule is the "tall" or "tower" shaped version......correct?

I think you need a faster router...

Speed - Apple Community

If the Time Capsule is set up as your router, the maximum speed through Ethernet would be in the 325-350 Mbps range.

OK, thanks for the info.

Try swapping the Ethernet cable from the ONT to the WAN "O" port on the Time Capsule. Use a CAT 6 Ethernet cable or higher rating.

If no help, the only other thing that might help out a bit would be to reset the Time Capsule back to factory defaults and then set it up again using the same name, password, same Wi-Fi network name, etc.

Using a standard cable modem, I can get about 290-310 Mpbs with a 500 Mbps Internet connection on my Time Capsule. Best case, the Time Capsule is going to top out at 325-330 Mbps.

As you know, the Time Capsule was designed more than 12 years ago when most Internet services were topping out in the 200-250 Mbps range.