I have a Verizon DSL modem with WIFI. My main computer is wired direct. The other day I was on the computer when internet started being very slow. I did a speed test and it showed 650 kbps. As it turns out my son was home and he was viewing a video on his smart phone via WIFI. As soon as he stopped my wired speed went to 2700 kbps. I thought that was strange so we did a test. I had him use his smart phone to access youtube while I was doing the speed test and it did cut the speed of the wired computer drastically. We repeated the test a few more times with the same result. It never used to do this before.
Is he watching a high definition video? I can say for sure that something like that definitely happened in the past, unless your DSL was running faster. You're sharing what is basically a total of 3Mbps across the two or more devices you have, and bandwidth is distributed not always in a fair fashion, but based on demand.
This is a new DSL modem. With the old modem I never noticed any wireless device having precedent over my wired computer. Yesterday I was watching a Amazon program on my Samsung TV (wired). My son came home and logged on to Youtube with his smart phone and the Amazon program just stopped.