It's probably the 2.4GHz WiFi transmitter interfering with the 2.4GHz mouse transmitter. You probably notice it during YouTube because it's constantly downloading. Try a wired mouse.
Interesting theory. The wired mouse is trouble free, but I figured that's because of a better sampling rate and less overhead over all.
Maybe I'll try a bluetooth mouse or some other frequency, or the laptop on fired Ethernet to see if the theory pans out.
