Slow LAN speeds with Ethernet and Wifi Combined

Packets going toward the Internet are sent to the TOPMost, working interface show in System preferences > Network.

Your Mac does NOT have the ability to combine more than one Ethernet connection into two, unless your Router device has explicit support for "Link Aggregation" protocols.

Your Mac does NOT have any hope of combining wi-fi and Ethernet connections into one. If Ethernet is available and TOPMost, Ethernet will be used for connection to the Internet. Wi-Fi can be on and can used for additional uses, such as location services, watch-unlocking, and continuity.

On older MacOS, Network Utility provided the ability to check the speed of each interface. at some point they moved it here:

/System/Library/CoreServices/Applications/Network Utility.app 

I am not sure Network Utility is still available but it has the ability to tell you your link speed, number of accumulated errors, and number of set and received packets, a strong indication of "working as intended".

Your symptoms strongly suggest you have Wi-Fi Interference. I do not think you are actually using Ethernet yet.

Use this article to "set Service Order" and place Ethernet at the top, wi-Fi SECOND.

Change the order of the network ports your Mac uses - Apple Support

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Your Mac looks at the IP address of where you want to go.

if the IP address is reachable on your current subnet, it is sent directly, using only the "Switch"portion of any Networking devices, not the "Router" portion.. That is, if your IP address has all octets in common with the destination IP address, when examined under the Subnet Mask for that interface, the packets will be sent directly. Example: from your Mac at 192.168.0.105, you wish to read files from a NAS at 192.168.0.250. Packets will be sent directly, if using the standard 8-bits subnet mask of 255.255.255.0.

All other packets must be sent to a Router to be forwarded, rather than sent directly.

If your network is complex, there may not be a path from Ethernet to a local NAS device, so paths using Routers would be examined. In general, the "shortest" path would be selected.

If the only available path to your NAS device is over Wi-Fi, then as I started out saying, it sounds like you have an interference problem on your Wi-Fi, and the Apple Watch has been implicated in such Wi-Fi interference issues.