Sudden Extreme Lag on WoW launchers (WotLK & Pandaria)

I've never had a problem with lag until today. Had the day off, so decided to login. However, the WotLK launcher started with major lag. I ran a backup copy of the WotLK launcher, and even a separate Pandaria launcher, but it still lags heavily. I tried several times throughout the day. It's even worse after logging in. My internet has been running at normal speed, so it doesn't seem like an internet issue. Just ran an internet speed test with the following results:

Download Speed
645.11 Mbps

Upload Speed
21.33 Mbps

Latency (ping)
9 ms

For a clear visual of what I have been dealing with, I've recorded & uploaded a screen recording of the real-time lag: https://youtu.be/YKtkdzRg0Pk