Is it possible that my MacBook Pro is causing slow internet speeds?

if your get very similar speeds on Ethernet And Wi-Fi chances are you actually did two tests on Wi-Fi.

Packets going to the Internet are sent using the TOPMOST, working interface shown in system preferences > network. Unless Ethernet is TOPMOST, you are not using it for your internet connection.

To get ethernet to the TOP, use the gear icon under the interfaces list and select 'Set service Order', then drag Ethernet topmost. When its cable is not plugged in, it will be not working and passed by and Wi-fi will be used instead.

staying with Ethernet for a while longer (because it does not have the complexities of Radio broadcast in your Network neighborhood) I want to ask about your Ethernet cables.

Please inspect all your Ethernet cables (mini to Router and if appropriate router to gateway.) Look through the clear end of each cable and check for EIGHT wires present, not just FOUR wires present.

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There are many different areas to probe to get the the bottom of this. I recommend you press on with investigating.

MacOS does not install older over newer. To go back, you need to have Trusted recent Backups, and completely ERASE your drive. Then any version of MacOS can be installed, and your files can be migrated from your backups. Straightforward but tedious.

Items that could be causing slowdowns:

Ethernet speed.

wi-fi speed.

background tasks eating too many resources and slowing your computer.

too many file-sync ing apps or the wrong ones

non-Apple File systems on external drives slowing file access down.

One inexpensive test that can be done is to restart and run in safe mode, and run your speed test from safe mode. This eliminates/implicates added software as potential causes.

On your top screenshot, you are showing VPN as a potential device. That item should be REMOVED, because just having it in the list constrains all your communications is un-intuitive and sometimes bizarre ways.

Select that VPN item in the network interfaces list and delete [-]

802.11ac MCS 2 NSS 3 80 MHz tops out at 290+ Mbps, so you’re getting what that configuration provides.

Given you have a Mac, consider acquiring WiFi Explorer or a similar app and viewing the local Wi-Fi activity.

What’s the Wi-Fi router box here? That it’s serving up 802.11ac implies either it or the client is old. Check its firmware, too.

An EtreCheck of the configuration is probably next, before a re-install.

Then it is possible you did test ethernet capability if Wi-Fi was off.

This has been a problem since dial-up modem connections. When users added a new interface, they assumed it would be used preferentially. but it will not be used AT ALL unless it is TOPMOST.

when using Ethernet, are you getting anywhere near the advertised speeds you pay for?

if you are, but WiFi is remarkably slower, we can look at your "network neighborhood' and see what might be going on there.

hold the Option key while you click on the Wi-Fi icon on the menubar. You gets a snapshot of current conditions like this older one. Screenshot, photograph, air transcribe the values and post back here for analysis.

the reason I am asking has to do with a physical performance step in Ethernet cables. Cables with FOUR wires can use BaseBand signaling, where one wiggle in voltage is seen as one Bit. These cables work fine up to 100 M bits/sec speeds, but NOT faster. (you are seeing an artificial limit at just under 100.)

to run faster, a modulated signal with much more complex patterns and multiple bits per signaling interval is used. It requires much more cable bandwidth, and will not run unless all EIGHT wires are present, and connected to the gold-colored contacts.

I posted just above that I thought having VPN as a listed network interface is a problem.

did you delete that yet?

about the Wi-Fi snapshot you posted, not sure which Mac its from:

You have marginal Raw signal (RSSI) of -71, where any worse means you Mac gives up and looks for another network to join. (the scale is reversed, where -40 is right beside your Router, and -75 is completely unusable.)

Noise of -87 is acceptable, but could indicate your neighbors are using adjacent channels. -95 is really good.

Your transmit rate of 292 says that speed restrictions are not being caused by Wi-fi Hardware, because you are running faster than your reported Internet speed test speeds.