I first heard about Yii on March 19th, and by the end of April we had migrated our website from Drupal to Yii. Yii was a big improvement in clarity and speed.
These charts show that the Yii performance benchmark using Hello World really does translate to real-world performance.
There are a lot of Web 2.0/social sites that started on Drupal and have outgrown it. I hope that many sites running Drupal or an older PHP framework see this Yii versus Drupal performance comparison and consider switching to Yii.
Please share or tweet this so more people find out how well Yii scales to a site with more than a million unique users. If you have questions, just ask me.