Gaming using 2-port powerline: Lag spikes only when YouTube is running?

My S/O and I are currently using a 2-port powerline adapter, and we get speeds around 30up/30down with ~12 ping. It seems that our setup works completely fine except in the situation where someone is watching YouTube while the other is gaming.

Things that didn't cause lag spikes:

• Playing CS:GO at the same time
• 1 person playing CS:GO, the other watching Twitch in "Source" quality
• Both watching YouTube videos at the same time
• Both watching Twitch streams at the same time

Things that did:

• 1 person playing CS:GO, the other watching YouTube in any quality (tested in 1080p, 720, 360)

Does anyone know why this happens? It seems pretty bizarre to us. Thanks!