Internet Speed Slow When Ethernet and Wireless are Connected

I'm at a loss with this problem. I have a Motorola SB6141 modem connected via ethernet cable to an RT-AC66U. The AC66U is running Merlin's 380.57. From the router there are connections to my laptop running Win 10, a telephone modem, powerline adapter and to a switch with 5 connections elsewhere. Everything has been working fine for both wireless devices (phones, laptop, tablet) and ethernet connected. This seemed to change when my wife got a new Laptop.
I was online this AM with good...

Internet Speed Slow When Ethernet and Wireless are Connected