Originally posted by 2slow2handle:

<— This is the sidebar. I have ~200 subscriptions and opera just freezes during loading all that stuff

Okay, I didn't like that list anyway :3
Add this as user CSS:

#guide-subscriptions-container {display: none;}

Thanks. It works.

Quick tutorial for newbies:

1. Right-click on YouTube
2. Edit site preferences...
3. Display
4. Create a file with path said in My style sheet
5. Add this to created file

#guide-subscriptions-container {display: none;}

6. Save

Same here.

The lag is caused by YouTube's new layout JavaScript.
This may be because of bug or because Opera 12 JS engine is just a lot slower than FF for example.
Sadly it's not possible to use YT with JS turned off.

I hope we will find a solution to this because I don't want to switch to other browsers just yet.

EDIT: I'm quite sure it's a bug because people on high-end machines are also lag when opening YT pages.