I'm running DD-WRT on my router (Linksys EA8500) , and my 5ghz band is running great, but my 2.4gz band has extremely handicapped bandwidth.

I have a bunch of IoT/Smarthome devices connected to my 2.4ghz band...
Most of them "Sip" bandwidth, however and shouldn't really be causing the loss of bandwidth I'm seeing.

When I run Iperf3 server on my router and client on my phone... I'm finding transfer speeds of about 1Mbits/sec
The same test while connected to 5ghz band is like 194Mbits/sec.

I checked my router's bandwidth usage, and it seemed to average around 10MBits/sec

I decided to temporarily change my wifi password on my 2.4ghz band, so my phone would be the only device connected to that band. Interestingly, Iperf3 now is showing average speeds of about 45.2Mbits/sec

Something doesn't seem right here.
With my phone being the only 2.4ghz client... its able to get 45Mbits/sec.
But when the band was full of clients, it was still only using 10Mbits/sec, but didn't seem to have any bandwidth left-over???

Is this an issue specifically with WIFI networking where not just the raw data bandwidth limiting throughput... but also the number of devices, irrespective of how little data they might be using? In other words, it almost seems like all these devices being connected, "Consumes" a certain amount of bandwidth regardless of whether or not those devices are transmitting and saturating their individual connections?

Turn off QOS?

Reconnect the devices with the new password one at a time. It's probably cheap chinese shit malfunctioning.

Or someone was using bandwidth because they had the password.

Check your DHCP lease and connections list on the router once in a while and learn what is yours so you can notice what isn't.

maybe it is the devices themsevles.

I may be remembering this wrong, but 2.4 ghz will drop down to the lowest speed due to backwards compatability.
so most people don't have this issue due to new devices, but some people have older stuff on their networks.

B and G wifi devices still connect and will drag your network down because it falls back to the oldest standard when connecting and forces all devices to that.

Define 'a bunch' of devices on your IOT network. 10? 20? 50?

13-15

6 Wifi Lightbulbs
4 Wyze Cameras
1 Wifi LED Controller
1 Thermostat
1 Ring Camera
1 Printer connected to a mini-router in AP mode.

I was able to figure out part of the issue.
I have a bunch of these devices connected to Home-Assistant installed on a Raspberry pi.
I had the Wyze Cams not only stream to an NVR... but I also added a Camera Integration to Home-Assistant. I was under the impression that this integration would ONLY use bandwidth when I click onto the Camera in the Dashboard application, to view the stream. Apparently, I was wrong, and it seems to draw a constant stream from the Cameras... forcing the Cameras to basically stream locally twice.

If only there was a way to force these Cameras to stream using Multicast... so that multiple devices can view the same camera footage without duplicating the bandwidth usage.

I don't know if this works for you, but wyze app has a "sharing" feature. lets you share the camera between devices. I use it so both my phone and the wife's phone can view the same camera.

-- this --

Some routers will do the drop down in speed of the lowest connection for compatibility issues.  One way to work around it is to have a dedicated 'slow' wifi to handle the devices that don't require the bandwidth.  I have a few devices that need to 'phone home' but don't need the necessary bandwidth to hook up to the high end wifi.   I have a few old routers that I converted to bridges to handle that hardware.

I'm aware of that feature. I have an alternate branch of firmware installed on my Wyze Cams that allows them to stream in RTSP format.
I run TinyCam Pro on an Android box, which serves as an NVR... somewhat unreliable, but better than nothing until my wife and I Can get around to getting Power-Over-Ethernet cameras installed...

Have you run a sniffer like wire shark to look for the top talkers?

I bet those IoT devices are dumping telemetry data back to the mothership and that is eating your bandwidth.

802.11 in general (i.e. all WiFi). All devices will initially connect at the what is known as the basic rate. For 2.4 GHz this basic rate is 1 Mbps (because that's what it was for 802.11b). For 5 GHz, that basic rate is 6 Mbps (because that's what it was for 802.11a). What happens is any time the AP beacons or needs to communicate with a device it drops down to the basic rate which begins to eat up more airtime as the number of devices increase. Also, all beaconing for all SSIDs is performed at the basic rate. Now with most devices (and I assume with DD-WRT but I've not messed with it in a decade) you can change the basic rate. The typical suggested minimum basic rate for 2.4 GHz devices is 11 Mbps (which essentially knocks most 802.11b support out) and 6 Mbps for 5 GHz (though some go to 12 Mbps).