Thanks for the update mstrozier.
Ya we are seeing the problem with the Pi3 as the muxing of the USB ports to a single USB 2.0 port. To quote the manufacturer
The USB host port inside the Pi[3] is an On-The-Go (OTG) host as the application processor powering the Pi, BCM2835, was originally intended to be used in the mobile market: i.e. as the single USB port on a phone for connection to a PC, or to a single device. In essence, the OTG hardware is simpler than the equivalent hardware on a PC.
as for the pi 4,
For the Pi 4, a fully-featured host controller drives the downstream USB ports. Downstream USB is provided by a Via Labs VL805 chip - that supports two USB 2.0 ports and two USB 3.0 ports. This is connected to the BCM2711 SoC using a PCIe link, which is extremely fast. Therefore, the Pi 4 does not have the speed constraints of previous models, which means very fast datarates, especially when using the USB 3.0 ports.
you can read more here:
https://www.raspberrypi.org/documentation/hardware/raspberrypi/usb/README.mdThere are a few things we found with the Pi 3B+ during our investigations (these are not a guarantee get it working at higher speeds, just finding we are documenting and though we should share)
1> the Wifi is a separate chip than the Ethernet over USB2.0 Microchip LAN7515 chip. Even though you may get better speeds wired than wifi, your eating USB bandwidth if used wired.
2> There is an issue with USB 3.0 hubs in conjunction with the some USB devices and the Raspberry Pi. A bug in most USB 3.0 hub hardware means that the Raspberry Pi cannot talk to full- or low-speed devices connected to a USB 3.0 hub. USB 2.0 high-speed devices, including USB 2.0 hubs, operate correctly when connected via a USB 3.0 hub so avoid connecting low- or full-speed devices into a USB 3.0 hub. As a workaround, plug a USB 2.0 hub into the downstream port of the USB 3.0 hub and connect the low-speed device, or use a USB 2.0 hub between the Pi and the USB 3.0 hub, then plug low-speed devices into the USB 2.0 hub.
3> because everything is handled by the SoC and a single port USB. limit the pi to as little services as possible and we even recommend overclocking if you are able. The Pi USB2.0 bandwidth is directly proportional to how much load the Pi SoC can handle. If you can disable Xwindows and run via SSH, you will get better performance.