Why is my upload speed on my mac so much slower than on the windows VM it hosts?

I suggest that you start by running a traceroute for both when you are running macOS natively, and then, inside the Windows VM. Are you using Parallels or VMware? Regardless how do you have their respective network option configured for the VM guest? Do you run a software firewall on your Mac? ... or any type of anti-virus app?

For macOS, you can use the Network Utility to run the traceroute. For Windows, you can use the command prompt. I suggest that you run the traceroute to the Google DNS server @ 8.8.8.8 ... or choose any destination you want as long as it is consistent for both platforms.

Look at each of the hops, especially the first few. Note the timing to see if there are any bottlenecks.

I can’t see how to run the windows one natively from the mac though, how do you do that?

When in the Windows VM, open a Windows Command prompt, and then, enter: tracert 8.8.8.8 <return>

Here's what you want to look for, using traceroute, from either platform:

• Increasing latency towards the destination: If you find a sudden increase in latency at a particular hop and it keeps increasing to the destination, this would indicate an issue at the hop with the increase.
• High latency in the middle of the trace, but not at the beginning: If there is high latency at a particular hop, but it returns to "normal" there after, it may just be that the router at that hop is adjusting to a lower priority. This does not indicate an issue.
• High latency in the middle and remains constant: This would not be an issue.
• High latency at the beginning hops: This would indicate an issue with the local network.

See if these help.

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You are welcome. I think you are being a bit harsh on yourself. Stuff happens.