100 Mb wired download speed vs 20 on WIFI

Hold down the Option key while you click on the Wi-Fi Icon on the Menubar. You get a display similar to this old one:

what values do you get for PHY mode and channel?

RSSI or signal strength or Signal-to-noise?

Transmit Rate?

and very important, how many other networks do you see?

You are using channel 35 in the 5GHz band, and a 40MHz wide channel. The fastest baseband speed on that channel using ONE antenna is 390 Mbits/sec, so your transmit speed is already four clicks down from the fastest one-antenna speed, and multiple-antenna speeds are not being used. This is likley due to interference on that or adjacent channels.

Your signal strength in your current location relative to the Router is less than excellent. signal at -76 less noise at -95 yields a signal-to-noise of -19, fair.

The other issue that comes into play with so many other networks nearby is signal spread. Your signal centered at channel 35 also spreads up and down the spectrum (especially using 40 MHz wide) to use portions of everything from the low end to beyond channel 50.

Looking at the exact channel conflicts is interesting, but to get a realistic picture you need a tool that shows the spread. Each nominal channel also sends data on adjacent channels as well, and can be influenced and interfered with by other Routers on nearby channels.

WiFiExplorer is an inexpensive tool I have used (and it has a three day free trial) that graphically shows the spread. Here is an example of its output graphs:

(click to expand)

You may also want to look at moving your Router, or adding another Router to build a multiple-Router connected network.

The signal level (and your ability to receive it without interference) goes slowly down as the frequency gets higher.

I think you may need to move your Router closer, or add another Router to your Network.

In a Mac pro silver tower, the antennas are partly shielded and reception is less than excellent. In a Mac Pro cylinder, reception should be great.

In a MacBook of any description, the antenna leads go through the hinge area, and are subject to abrasion damage -- so it is possible that one or more antenna lead is broken or shorted.

If you can, you could try doing a test within one or two meters of the Base Station. Your RSSI (raw signal) should improve to better (lower - numbers) than -50. I would also expect in this case that your Transmit rate would then exceed the fastest baseband speed of 390 M bits/sec, indicating you are using multiple antennas.

It is possible that sometimes you are making the connection to your 2.4GHz network with the same name. That will give you slower speeds because it conflicts with another strong network on channel 11. It is sometimes helpful to give the two networks slightly different names (changing them in your Router). Then your computer can be told to only join one and not the other.