You'll have to wait a bit until the YT download extensions are updated for the new interface. Meanwhile, you can use http://keepvid.com/ or other websites.
Please specify some websites that takes ages to load so we can have a look at what's wrong. General criticism isn't helpful if we can't reproduce problems. It's not that we didn't test this browser version before releasing it.
You are, of course, free to revert to an older version. However, we don't recommend this because we do care about security.