Router Internet VERY SLOW!!!

I have to say that, of the possibilities you've listed below, I have none of them - and I have no idea why the severe drop in my router signal is occurring. As I've said, I am only three feet from the router. This problem began last Fall for no apparent reason.
I have no other devices that would interfere with my signal. No HAM radio operators nearby.

RickC said:

I pay for a 150Mbps service.
My ethernet-connected test shows 163.63Mbps.
My wifi-connected test shows 65.62Mbps (using 2.4GHz connection, 40 feet between laptop and router, 6 other wifi devices connected to the same router, all using 2.4 GHz).
Attachment 193347
I consider these results fairly typical.

By comparison, you're paying for a 100Mbps service.
Your ethernet-connected test shows 123.01Mbps.
Your wifi-connected test shows 48.56Mbps but you say it's normally 15Mbps to 25Mbps.

IMO there's 2 issues:
1. Why is your wifi connection speed relatively poor when your wifi-connected device is only 15 feet away?
2. Why the wide variance between 48.56Mbps and 15Mbps?

Issue 1: Maximizing connection speed
Interference - Are there any other devices using 2.4GHz? (Cordless phones, baby monitors, VOIP/wifi phones, Bluetooth devices, nearby car alarms, satellite TV decoders/video senders, etc? Are there any nearby neighbours using the same wifi channel?)
Obstruction - Is this clear line of sight or are there any walls in between.
Absorption - Fish tanks, people, radiators (basically, anything with water in them)
Swamping - How many other wireless devices are in use (laptops, cell phones, tablets, etc.)
Wifi device drivers

Issue 2: Consistency/less variance
Similar to the mainly static conditions above but more variable, i.e. people/animals moving about.

Hope this helps...

Netgear, Model R6300v2

Samuria said:

Make and model of router

In the manual http://www.downloads.netgear.com/fil..._13Aug2014.pdf

Page 25
Mode
. For 2.4 GHz, Up to 217 Mbps is the default setting, which allows 802.11n and 802.11g
wireless devices to join the network. The other settings are Up to 54 Mbps, and Up to 450
Mbps.
If thats set wrong it will slow it right down
page 32 set mtu Maximum transmission unit - Wikipedia

Nisko said:

Netgear, Model R6300v2

Now that you have established the speed problem is not with your ISP but with your very Router and only due to its wireless portion, the statement that this has been happening since last fall and no configuration changes were undertaken leads me to think that you may in fact have a hardware problem with said Netgear R6300v2. If still in warranty you should seek a replacement.

Here is my Setup Page - I don't understand everything you wrote so that's why I'm sending you this screenshot.

Samuria said:

In the manual http://www.downloads.netgear.com/fil..._13Aug2014.pdf

Page 25
Mode
. For 2.4 GHz, Up to 217 Mbps is the default setting, which allows 802.11n and 802.11g
wireless devices to join the network. The other settings are Up to 54 Mbps, and Up to 450
Mbps.
If thats set wrong it will slow it right down
page 32 set mtu Maximum transmission unit - Wikipedia

@Samuria, Where do we stand? Are my settings correct?

Nisko said:

Here is my Setup Page - I don't understand everything you wrote so that's why I'm sending you this screenshot.

That page shows not much sadly. Did you look into warranty or was the wireless router also exchanged by the cox tech?

Your wireless router provides two bands of operation: 2.4GHz and 5GHz, do you know which band your clients are connecting to? You could name your SSID's for the 2.4GHZ band and 5GHz band differently so that you specifically connect your clients to the 5GHz band preferably. Also you should set the 2.4GHz and 5GHz operating channels manually after doing a site survey with some software which gives you an idea of what channels are less polluted in your area of operation. If you do not plan to connect wirelessly older 802.11b/g devices, then 802.11n/ac mode only should be set for the radios in the router as this will improve performance.

Set it to 450 for 2g and 600 for 5g save and try speed test again test connected to 2 and 5

Tried it - it didn't help. I wonder if there's a 'gadget' that can detect radio waves that might be interfering with my router. Perhaps even pointing in the direction they're coming from!

A diagnostic tool that you ask about is not easily found due to cost but you can check yours and neighbors WiFi signal strength and more with a tool such as Acrylic WiFi Home.