slow wifi on MacBook Pro

Hold down the Option key while you click on the Wi-Fi Icon on the menuBar. You get information similar to this older report:

What do you get for PHY mode and Channel?

what do you get for signal strength, signal-to-Noise, or RSSI? at what distance from your Router?

What do you get for Transmit Rate or Tx rate?

and very important, how many other network-names do you see?

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802.11ac is only available on the the 5GHz band, so you must be using your reported channel 149. Without a lot of neighbors present, there will be no interference from other networks.

Your Router likely set up an 80MHz channel and is likely using two antennas to double the baseband speed of nominal 433.3 to get nominal 866, reported as 878.

All of that looks like it is working just fine. If you wished to do so, you could adjust the settings in your Router to have it choose "automatic" channel selection. After each reset, it sniffs the airwaves and chooses the lowest numbered channel (lower numbered channels have slightly more 'reach') that still provides best transmit rates with least interference.

In Airport Utility, that setting is in the Wireless pane, Wireless options button, and choose "Automatic" from the list rather than a certain channel for your 5GHz Network.

When you name the two bands separately (2.4GHz and 5GHz) you are providing two over-the-air subnetworks, and you can choose to join either sub-network. If you name them both the same and use the same encryption and password, they merge into one network and each device decides on its own which band to join.

If you have both sub-networks listed on the preferred networks pane, moving a little too far from the Router may flip you to the other, slower network.

to join only the faster 5GHz network, delete your 2.4GHz network from this pane, and check the "ask to join new Networks" box on the previous page:

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Running an Internet speed test (readily available WITHOUT additional software) from many different Internet sites near you will test the complete link from a test site relatively nearby, through your Router, over your airwaves and into your computer to your software. These tests bypass many possible items that could slow your speed way down, (such as problems with Domain-name service) and give you a good idea of attainable speed over your connection. (Obviously a test from New York with you in the UK will not fully accomplish this.)

A search for "Internet speed test" should provide several such test sites provided by vendors who hope you will choose their service over the one you currently use.

One important item to look for when your speed seems slow is so-called Anti-Virus software. Your premium quality Macintosh computer is already well-protected with built-in anti-Virus features, and third-party software will just slow it down and make it unstable. You should NOT be running third-party anti-Virus on your well-protected Mac.

what channel is your Router using?

are the two networks you see yours or your neighbors' networks?

Have you performed a Network Speed test (provided by many Internet sites)?