These freezes were caused by worlds using ProTV in particular. I'm unsure what other types are affected. But here's the easy reproduction steps:
  1. Go to a World using ProTV.
  2. Try to load a Stream on YouTube.
  3. See how hard your client freezes.
The URL my Video Player is auto-playing:
Here's my own World where it freezes. Test it out here:
Output Logs Here: