[SOLVED] Extremely High Ping when others are using the Internet

I don't think QoS will help here. 20 Mbps should be plenty for YouTube + a game. 1080p Netflix only uses about 5 Mbps, and I can't imagine YouTube is much different.

My guess would be the bottleneck is the wifi. If it's an older router which doesn't support n or ac, then it's probably g which means all devices slow down to the speed of the slowest device. I can see that easily slowing down throughput to 5 Mbps or lower - enough for a YouTube stream to impede other traffic. In fact OP should run a speedtest over wifi to see if he's actually getting the full 20 Mbps over wifi.

To OP: You say you're on a laptop. Take the laptop downstairs, and try plugging in with an Ethernet cable. Yes I know this won't work as a solution; you're only doing this to test. Go play your games, and see if you encounter the same ping spikes when someone else watches YouTube. If the cable eliminates the problem, then you've got several possible solutions:

1) Download WiFi Manager on your Android phone (I'm sure iPhones have a similar app available). Use it to see what wifi signals are in your house. Note the channels of the networks with strong signal strength. Now go into the router's settings and set its wifi channel to the least-crowded channel you spotted. You're trying to minimize interference so hopefully devices connect at a higher speed eliminating your problem. If it's a 2.4 GHz network, it'll look like you have 11 channels available (1-11). But in reality channel 1 covers frequencies from 1-5, 6 covers 6-10, and 11 covers 11-15. So there are really only 3 channels. Pick a channel (1, 6, or 11) accordingly. If it's a 5 GHz network (only available with n or ac), then you'll have a much wider selection of non-interfering channels.

2) Assuming your laptop supports n or ac, give AT&T a call. Complain that the router is slow with your brand spanking new laptop. A lot of times they'll replace your router with a newer model that supports n or ac.

3) If AT&T won't replace the router, you can buy your own router. Plug its WAN port into one of the AT&T router's LAN ports. Use the new router to set up your own wifi network. Make sure it's on a different channel than the AT&T router's wifi so they don't interfere. The person in the house watching YouTube will then be on the AT&T router's wifi. You will be on the new router's wifi on a separate channel. His traffic will no longer interfere with yours (at least not until 60 Hz 4k video streams become common - those are expected to take about 20 Mbps).

As for Solution 1, I have a application to check wireless channels around my area and changed the channels back and forth and nothing gets better. Solution 2 is not viable as I have a G. And Solution 3 is connected to Solution 2 so it won't work.