Internet speed

Internet speed has NOTHING to do with your iMac. It has everything to do with your modem and router and your home network setup. The first thing to do though is:

1. Unplug modem & router for 2-3 minutes.
2. Restart the modem and router.
3. Restart the computer in Safe Mode and then restart normally and test.

If your iMac is in the same room as your router, then you are fine. However if your iMac is located in the opposite end of the home or somewhere between then you may have a home networking issue that you would need to resolve. What that means is you may have a quick speed coming into your home but if you use Wi-Fi the signal weakens the further away from the router your devices are.

If the steps (1,2,3) don't help, you also need to reset your DNS settings as you may be using the defaults from your ISP which can make a home network problematic.

Not sure how you are measuring at the gateway and at the computer? Are you wireless or wired? You can run an Ethernet cable for quite long distances with little signal loss. If the computer is not moving, run a wire, under the rug, over the door, out a window from the basement, whatever, just run a cable. Those 2012 iMac models have Gigabit Ethernet ports.

If you are using WiFi the 2012 iMacs have the older 802.11 a/b/g/n (ie WiFi 4) which does indeed have EVERYTHING to do with your wireless internet speed. Theoretical max of that standard is 480 Mbit/s, but real world performance is probably going to be half that if you are lucky, or nearly 200mbps which is what you are reporting.

Based on the information in your post, I am assuming that your iMac is connected to your network's wireless ... correct?

If that's the case, please provide the following from the Wi-Fi icon on the macOS menu bar. Note: Hold down the Option key before clicking on the icon.

• Security = ?
• Channel = ?
• Country Code = ?
• RSSI = ?
• Noise = ?
• Tx Rate = ?
• PHY Mode = ?
• MCS Index = ?
• NSS = ?

Also, do you happen to use Bluetooth mouse and/or keyboard with your iMac? ... or any other Bluetooth devices that are operating nearby?

Thank you! Now let's go over those numbers ...

• Security = WPA2 Personal << This is what you want as it provides the best throughput performance over wireless
• Channel = 48 (5 GHz, 40 MHz) << Your Mac is connected to the lower power band channels of the 5 GHz wireless network that is using 40 MHz-wide channels. Ideally, in the US, you would want to connect the Mac to channels in the upper region of that band ... somewhere in the 149 - 161 range.
• RSSI = -29 & Noise = -88 << We use these values to determine the signal-to-noise ratio (SNR). SNR is one measure of signal "quality." SNR = RSSI - Noise. Here SNR = -29 - (-88) = 59. Anything above 40 would be considered "excellent." ... and this should be the case if your iMac is only 3' away from your wireless gateway.
• Tx Rate = 162 Mbps << This is the instantaneous signal transmit rate at the time of the reading as seen by your Mac.
• PHY Mode = 802.11n << As pointed out by the others, your Mac is limited to this standard. This standard has been superceeded by 802.11ac, and now 802.11ax which both provide greater bandwidth. However, it is important to understand that both the wireless client and the wireless router must support the same standards.
• MCS Index = 19 << This is the most telling number. Ref: MCS Index - 802.11n and 802.11ac. Using the link I provided you, go down the HT MCS Index column until you come to the 19 value. You can see that it ranges from 0 to 23 (worst-to-best). Going across to the 40MHz No SGI column, you will see the value 162 (Mbps). The MCS Index is another method to determine signal quality. As you can see, what your Mac is capable of connecting at is not the best it should be able to. This could indicate a faulty AirPort card OR some type of Wi-Fi interference is nearby.
• NSS = 0 << Older version of macOS did not include this value. Regardless, that MCS Index Table does. NSS stands for Number of Spatial Streams. From the table, for Index 19, it shows that NSS would be: 3. That means the Mac's wireless connection support 3 streams ... the best it can ever do.

Since all of your Bluetooth peripherals would be operating in the 2.4GHz band, they shouldn't be causing any Wi-Fi interference.

I suggest that you follow rkaufmann87's advice to troubleshoot your gateway to make sure it is properly configured to provide the best wireless network ... and, if that doesn't resolve this issue, then you still have a few choices ... and those are: 1) Consider getting a USB wireless adapter. These come with better 802.11 standards support and may overcome any of the limitation your Mac's built-in AirPort card may have, 2) Since your Mac is only 3' from the gateway, as Glen Doggett has suggested, connect your Mac to the gateway with an Ethernet cable, and 3) Consider upgrading your Mac to a newer model.

Just to add, a 2012 iMac uses 802.11n Wi-Fi so the Maximum data rate (theoretical) is 450 Mbps. So 200 Mbps is not out of the question.

I would ask AT&T but most 1Gbit plans get the maximum speed using an Ethernet connection. Do they claim 1Gbit Wi-Fi?

I think if you can provide that info I asked for, we can at least establish what your iMac is currently capable of. That way we can rule out if it your Mac or not that is the primary culprit.

You may have quite a wait if you want to order another iMac. The M3 were only released a couple of months ago so new model iMacs may be 18 months or more.