How many devices in the Small Business network? What sort of devices: computers, printers, storage (NAS), mobile?
Mix of wired and wireless?
Make and model: modem, router (or modem /router if combined)?
The Router's logs, if available and enabled, may provide some clue.
Also check the router's configuration pages for allowed number of network devices, assigned IP addresses (dynamic and static), connected devices, MACs, restrictions, QoS, and similar configuration settings that impact individual systems on the router's network,
DHCP environment? Any Static IP's?
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Take a look at Task Manager, Resource Monitor, and Process Explorer on the slow upload systems.
Note: Process Explorer. Microsoft, free.
The objective being to determine what resources are being used, to what extent (%), and what is using any given resource.
Use all three tools but only one tool at a time. Compare a fast upload to a slow upload PC to look for differences.
Run the tools while not uploading to get a baseline understanding of the test/slow upload PC. Then, leaving the tool window open and viewable, do some uploads as usual.
Again watch for changes.
And take a look at a slow upload PC before (if possible) using the USB NIC. Do that before (USB NIC not installed/ PC slow upload) and after (NIC installed, upload speeds okay) comparison.
I would also start comparing the results of "ipconfig /all" and "arp -a" for slow and fast upload systems. Do the same when the computer is slow and then again when fast.
Keep track of IP addresses assigned to each network device. Look for conflicts or some other IP address error.
Sketch out a network diagram showing all devices, IP addresses, MACs, and so forth. Map out how the problem is moving about and look at the diagram. May be some clue therein.
See what you can find.
Be methodical, keep notes, use the diagram. If you have not already done so then write down as much history as you can remember.
Key is to discover what is different (or becomes different) about any PC that starts having slow uploads. And becomes via whatever means, a fast upload PC again.
My thought is a IP address conflict of some kind. Possibly related to a DHCP IP address that would be moving around amongst PCs on a network. Sooner or later comes into a conflict again.