I can’t say it’s a Safari 4 feature, I tested Firefire 3.5 beta, they can achieve it. But it definitely fixed a serious problem in Safari 3. If you have tried some resource killing web site, e.g. HD flash movie page, Facebook games like Restaurant City, you will find the CPU usage can be pushed up more than 100%, if you have dual core processor!! And my MacBook Pro can be heated up to 88 degree Celsius (why I hate Flash!). The worst is if you only a new tab and browsing page or even minimize Safari 3 into Dock, the CPU usage is still high as before!
Now I found Safari 4 fixed this issue very well. Now I can’t use my MBP to fry egg! 🙂
Restaurant City in foreground
Restaurant City in background