As far as I can tell, based on your network diagram, your speed problems probably come mainly from the mesh wifi.
If you're interested in testing your wifi speed without relying on an internet service, you could use this
this app I wrote last year to do local speed testing. You can connect any modern web browser to the app running on a Windows machine, and test download speed only (transfer from the app to the web browser).
The web port the app listens on can be configured by pressing the Web Port button. Then you push "Start Temporary Instance" and you can connect to it with a browser as seen below. You'll need to know the IP address of the PC you are running the app on, noting that wifi and wired network interfaces would have different IP addresses. You also may need to edit Windows firewall settings to allow the incoming connections if you aren't prompted at startup.
With two windows PCs, you can click the
Duplex Test button within the app use a more advanced speed testing method which offers tests of upload, download, and duplex (both up and down simultaneously).
To do a duplex test, start temporary instance on one of the windows PCs, then use the Duplex Test button on the other. You should see the other machine appear in the Remote Host list and be able to perform tests as seen here:
As you can see, duplex test performance can be pretty sucky and this is especially true over wifi (the above is actually wired).