Ethernet + WiFi = Slow Network

Hello Jamie Cox1,

Thank you for using Apple Support Communities!

I understand from your post that having both Wi-Fi or Ethernet internet enabled the internet speed is very slow. In a situation like this prioritizing your network locations may help:

How to use network locations on your Mac - Apple Support

Best Regards.

Jamie Cox1,

You may want to consider testing the issue while your Mac is in safe mode and/or in a new user account. This will help determine the cause of the issue. The following articles have the steps you will need to test the issue in both safe mode and in a new user account: 

Use safe mode to isolate issues with your Mac - Apple Support

How to test an issue in another user account on your Mac - Apple Support

Regards.

There’s probably more than one IP router present here, and the network segments are not configured in separate subnets. Or that Asus box is doing something weird, and you’ll want to check for a firmware update, there. Reduce this to one IP segment by switching a Wi-Fi router into an access point (what Apple calls “bridged”), or set up separate IP subnets and establish IP routing. Or maybe switch the Mac to an ad-hoc local network, though I’ve not tried that.

I’d be inclined to swap the router first, but hardware problems and bad cabling are absolutely in play here. The last few times I saw this have been with older OS X software, and with loops in the network cabling. Here, I’d see if switching the Mac Wi-Fi to an ad-hoc network and off the router Wi-Fi and this for the watch logins would work around any firmware problems (multicast mishandling) that might lurk in the router.

Duplicate names arise when there’s a Mac with multiple links to and assigned IP addresses within the same subnet.

If you’re seeing that, switch to different subnets.

This all reeks of a subnet routing issue with this gear, or this network.

Edit: "The issue with the duplicate name on the network is unchanged. That is a continuous, repeatable problem on all Macs with more than one network interface in the same network subnet

There. I fixed it for you.

and it is not a problem - it's an expected situation. By your setup, your Mac does appear twice in the same network.

This is generally a no-no, and the fact it works at all is somewhat surprising, and Mac actually be related to your slowdown.

Generally, if you have two links in the same subnet they would be bonded together to form a faster virtual link.

If you are using two links to the same subnet, one of them should have no router configured so the OS does not try to use that link for non-local traffic. In this setup that requires manual configuration unless you can configure the DHCP server to not hand out a router address to the wi-fi interface (unlikely).

I’d fix the invalid network design here, but whatever works. Two IP routers means entirely disjoint networks (and this Mac breaks that, by connecting to both), or configuring two subnets, or switching one IP router to bridged / access point mode and eliminating all but one IP router.

Okay, if the ASUS is set bridged and you’re getting to a DHCP server, that’d usually mean one IP router, and one IP subnet would be typical.

Check the ASUS firmware, too.