Internet Speed

I have 2x 27" iMacs connected to a BT router supplied by BT Fibre. Both similar specs bur Mac 2 is only 2 years old, Mac 1 is 6 years old. Mac 1 is hardwired to Router with approx 15 metres of cable and using Think Broadband i get 500mbps, Mac 2 is hardwired to Router using only 1.5 metres of cable and using Think Broadband I only get 100mbps. Downstairs on my iPad on WiFi i get 250mbps.
What is the problem?

It is possible the cable to your Mac 2 is faulty and limiting speeds to 100 megabit/sec.

Are all ports on your router gbit capable?
Is the Mac also connected via wifi, if so is ethernet at the top of the list in the network 'service order'?
Are they any lights on the ethernet port indicating link speed?
On the Mac, what does ifconfig from a terminal window show?

en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
options=40b<RXCSUM,TXCSUM,VLAN_HWTAGGING,CHANNEL_IO>

prefixlen 64 secured scopeid 0x6
inet 192.168.xx.xxx netmask 0xffffff00 broadcast 192.168.xx.255
nd6 options=201<PERFORMNUD,DAD>
media: autoselect (1000baseT <full-duplex,flow-control>)
status: active

Edited by smouty (Sun 24-Apr-22 18:39:04)

Or Mac 2 is set to 100 meg in the Network settings in System Preferences.

Take a look in System Preferences->Network

Click on the Ethernet connection in the list and then click on Advanced, then the Hardware tab.

It should be set to Configure: Automatically, it will also show you there whether the speed it is negotiating with the router's switch is 100 or 1000.