I'm not a mac person but currently the OP has

iphone at full speed

imac with big sur running at 100mbit/s or less

there may be potential cable issue, or hardware issue or big sur having issues (some users have had had speed issues)

@MysticLady Which version of Big sur are you on?

In reply to a post by Taras:

there may be potential cable issue, or hardware issue or big sur having issues (some users have had had speed issues)

@MysticLady Which version of Big sur are you on?

I'm a multi OS user so use Macs, PC's and some linux.

I had big sur and had some issues with networking over wireless, some days 500+ others 250 or less, it would then resolve itself on a few reboots. Now it's a lot more stable on the newer versions but I strongly recommend upgrading to Monterey if you have no compatibility issues with other apps, make sure to test this.
Do you happen to have another mac or PC lying around you could run a test on over ethernet just to give us an idea?
Also not sure if anyone said but have you tried disabling your antivirus to run the tests as I have enterprise grade antiviruses and they really suck all the resources.

as the others said its probably the ethernet cable causing a problem, i would be tempted to say so myself.

Let us know how you get on

I'm hoping the OP will come back tomorrow with the cable test - so we can rule that out, and then on the Big sur version. As some BigSur versions have, as you said had issues with networking. If Monterey is free of wireless networking issues then this is the way to for the op as long they don't face compatibility issues

Apple BigSur 11.6.1

I don't believe it's the cable. Even standing next to the router and running the Fing app on my iPhone 7 it is reporting WiFi speeds of 90.6 Mbps down and 103.0 Mbps up.

I also tested the speed on a MacBook pro next to the router and got a slightly faster down rate.

Running a speed test via the Linksys app itself reports speeds of 500 Mbps up and down. Something isn't right with the way the router is configured perhaps?

I even called Apple yesterday and they said they could not see anything wrong with my iMac settings when sharing my screen. I spoke to a guy at Community Fibre too, but he didn't seem to have a clue.

I still have BT Wifi up and running, could that be interfering?

Here's what I get from ifconfig in Terminal:

lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
options=1203<RXCSUM,TXCSUM,TXSTATUS,SW_TIMESTAMP>
inet 127.0.0.1 netmask 0xff000000
inet6 ::1 prefixlen 128
inet6 fe80::1%lo0 prefixlen 64 scopeid 0x1
nd6 options=201<PERFORMNUD,DAD>
gif0: flags=8010<POINTOPOINT,MULTICAST> mtu 1280
stf0: flags=0<> mtu 1280
en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
options=50b<RXCSUM,TXCSUM,VLAN_HWTAGGING,AV,CHANNEL_IO>
ether 78:7b:8a:b1:df:46
nd6 options=201<PERFORMNUD,DAD>
media: autoselect (none)
status: inactive
en1: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
options=400<CHANNEL_IO>
ether 98:9e:63:44:8f:22
inet6 fe80::c7b:c609:603f:2c6%en1 prefixlen 64 secured scopeid 0x5
inet 192.168.1.83 netmask 0xffffff00 broadcast 192.168.1.255
inet6 2a02:6b62:20a1:0:1029:32f3:8c69:65d3 prefixlen 64 autoconf secured
inet6 2a02:6b62:20a1:0:c80e:7a62:54f3:fc5a prefixlen 64 autoconf temporary
nd6 options=201<PERFORMNUD,DAD>
media: autoselect
status: active
en2: flags=8963<UP,BROADCAST,SMART,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu 1500
options=460<TSO4,TSO6,CHANNEL_IO>
ether 82:22:05:43:38:00
media: autoselect <full-duplex>
status: inactive
en3: flags=8963<UP,BROADCAST,SMART,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu 1500
options=460<TSO4,TSO6,CHANNEL_IO>
ether 82:22:05:43:38:01
media: autoselect <full-duplex>
status: inactive
bridge0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
options=63<RXCSUM,TXCSUM,TSO4,TSO6>
ether 82:22:05:43:38:00
Configuration:
id 0:0:0:0:0:0 priority 0 hellotime 0 fwddelay 0
maxage 0 holdcnt 0 proto stp maxaddr 100 timeout 1200
root id 0:0:0:0:0:0 priority 0 ifcost 0 port 0
ipfilter disabled flags 0x0
member: en2 flags=3<LEARNING,DISCOVER>
ifmaxaddr 0 port 7 priority 0 path cost 0
member: en3 flags=3<LEARNING,DISCOVER>
ifmaxaddr 0 port 8 priority 0 path cost 0
nd6 options=201<PERFORMNUD,DAD>
media: <unknown type>
status: inactive
p2p0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 2304
options=400<CHANNEL_IO>
ether 0a:9e:63:44:8f:22
media: autoselect
status: inactive
awdl0: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu 1484
options=400<CHANNEL_IO>
ether 8a:a4:3a:2c:41:3c
inet6 fe80::88a4:3aff:fe2c:413c%awdl0 prefixlen 64 scopeid 0xb
nd6 options=201<PERFORMNUD,DAD>
media: autoselect
status: active
llw0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
options=400<CHANNEL_IO>
ether 8a:a4:3a:2c:41:3c
inet6 fe80::88a4:3aff:fe2c:413c%llw0 prefixlen 64 scopeid 0xc
nd6 options=201<PERFORMNUD,DAD>
media: autoselect
status: active
utun0: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 1380
inet6 fe80::6370:2f97:ec48:6195%utun0 prefixlen 64 scopeid 0xd
nd6 options=201<PERFORMNUD,DAD>
utun1: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 2000
inet6 fe80::22d6:74ad:8ce5:4348%utun1 prefixlen 64 scopeid 0xe
nd6 options=201<PERFORMNUD,DAD>

Edited by MysticLady (Fri 05-Nov-21 12:19:53)

In reply to a post by flippery:

In reply to a post by MysticLady:

iMac (Retina 5K, 27-inch, 2017) / BigSur 11.6

Connection is via WiFi to a Linksy router.

2.4g or 5g connected

I'm not sure - how do I check? I think the router is set to "mixed" by default.

Edited by MysticLady (Fri 05-Nov-21 12:27:52)

Mmm…that ifconfig output is not showing any network cable connected that I can see.

Can you disable WiFi on the iMac and please connect to the Community Fibre router using a direct network cable (Cat5e or Cat6 with RJ45 plugs) and redo both the speedtest and ifconfig command and repost the outputs. Thanks.

In reply to a post by MysticLady:

iMac (Retina 5K, 27-inch, 2017) / BigSur 11.6

Connection is via WiFi to a Linksy router.

2.4g or 5g connected

I'm not sure - how do I check? I think the router is set to "mixed" by default.

On mac to check what frequency you are connected to you hold the option key and press on the WiFi icon in the Menu Bar, it will display a load of information about the channel, frequency, width, among other things could you let us know what channel and all of that information.
could you also share the "PHY mode" it will come up near the bottom of the list.

Not sure if it would be beneficial to ask for the noise rating as well as it could help to see if there is channel congestion?

Using direct cable connection:

Speedtest.net:

Ping: 3 ms
Up: 89.38
Down: 94.99

ifconfig:

lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
options=1203<RXCSUM,TXCSUM,TXSTATUS,SW_TIMESTAMP>
inet 127.0.0.1 netmask 0xff000000
inet6 ::1 prefixlen 128
inet6 fe80::1%lo0 prefixlen 64 scopeid 0x1
nd6 options=201<PERFORMNUD,DAD>
gif0: flags=8010<POINTOPOINT,MULTICAST> mtu 1280
stf0: flags=0<> mtu 1280
en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
options=50b<RXCSUM,TXCSUM,VLAN_HWTAGGING,AV,CHANNEL_IO>
ether 78:7b:8a:b1:df:46
inet6 fe80::1008:cfe4:c747:3b0f%en0 prefixlen 64 secured scopeid 0x4
inet 192.168.1.91 netmask 0xffffff00 broadcast 192.168.1.255
inet6 2a02:6b62:20a1:0:145b:ee86:7523:ee50 prefixlen 64 autoconf secured
inet6 2a02:6b62:20a1:0:5f:c813:acae:8b31 prefixlen 64 autoconf temporary
nd6 options=201<PERFORMNUD,DAD>
media: autoselect (100baseTX <full-duplex,flow-control>)
status: active
en1: flags=8823<UP,BROADCAST,SMART,SIMPLEX,MULTICAST> mtu 1500
options=400<CHANNEL_IO>
ether 98:9e:63:44:8f:22
nd6 options=201<PERFORMNUD,DAD>
media: autoselect (<unknown type>)
status: inactive
en2: flags=8963<UP,BROADCAST,SMART,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu 1500
options=460<TSO4,TSO6,CHANNEL_IO>
ether 82:22:05:43:38:00
media: autoselect <full-duplex>
status: inactive
en3: flags=8963<UP,BROADCAST,SMART,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu 1500
options=460<TSO4,TSO6,CHANNEL_IO>
ether 82:22:05:43:38:01
media: autoselect <full-duplex>
status: inactive
bridge0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
options=63<RXCSUM,TXCSUM,TSO4,TSO6>
ether 82:22:05:43:38:00
Configuration:
id 0:0:0:0:0:0 priority 0 hellotime 0 fwddelay 0
maxage 0 holdcnt 0 proto stp maxaddr 100 timeout 1200
root id 0:0:0:0:0:0 priority 0 ifcost 0 port 0
ipfilter disabled flags 0x0
member: en2 flags=3<LEARNING,DISCOVER>
ifmaxaddr 0 port 7 priority 0 path cost 0
member: en3 flags=3<LEARNING,DISCOVER>
ifmaxaddr 0 port 8 priority 0 path cost 0
nd6 options=201<PERFORMNUD,DAD>
media: <unknown type>
status: inactive
p2p0: flags=8802<BROADCAST,SIMPLEX,MULTICAST> mtu 2304
options=400<CHANNEL_IO>
ether 0a:9e:63:44:8f:22
media: autoselect
status: inactive
awdl0: flags=8902<BROADCAST,PROMISC,SIMPLEX,MULTICAST> mtu 1484
options=400<CHANNEL_IO>
ether 8a:a4:3a:2c:41:3c
nd6 options=201<PERFORMNUD,DAD>
media: autoselect
status: inactive
llw0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
options=400<CHANNEL_IO>
ether 8a:a4:3a:2c:41:3c
nd6 options=201<PERFORMNUD,DAD>
utun0: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 1380
inet6 fe80::6370:2f97:ec48:6195%utun0 prefixlen 64 scopeid 0xd
nd6 options=201<PERFORMNUD,DAD>
utun1: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 2000
inet6 fe80::22d6:74ad:8ce5:4348%utun1 prefixlen 64 scopeid 0xe
nd6 options=201<PERFORMNUD,DAD>

I've also just just checked my iMac specs:

10/100/1000BASE-T Gigabit Ethernet (RJ-45 connector)

Wi-Fi
802.11ac Wi-Fi wireless networking
IEEE 802.11a/b/g/n compatible

Bluetooth 4.2 wireless technology

Edited by MysticLady (Fri 05-Nov-21 12:53:17)

In reply to a post by MysticLady:

media: autoselect (100baseTX <full-duplex,flow-control>)
status: active

You're connected at 100Mbps - most likely a cable issue as has been said before.