We challenge you to find a framework that has better performance than CodeIgniter.

[eluser]Twisted1919[/eluser]
Yii's documentation is not like you are used to on CI, but every method in class is very well explained,
you can start from here: http://www.yiiframework.com/doc/guide/ and of course you have all the class reference here: http://www.yiiframework.com/doc/api/
Now, i will give you a few reasons( that really matters )why you can go with Yii over CI. Don't get me wrong, i still develop with CI on some projects, because, well CI is what i know best, still learning Yii.
1) Themes support - I like having a theme for admin panel and one or more for frontend.
2) Native HMVC - Well, i cannot create a project without modules. Sure, CI has HMVC but is not in the core
3) Authentication and extensible RBAC - CI doesn't have either of those.
4) A reliable session class
5) Cache using file/apc/db etc
6) A true Active Record class (CI's AR is actually a query builder not Active Record perse)
7) Transactions that works (In CI, that's something that doesn't work)
8) Support for $_GET and the way Yii let's you define the routes using $_GET is just great.
9) Assets manager
10) The validation classes are just great, something like CI does, but in a smoother and extensible way.

You say that people left Yii for other frameworks, well, isn't this normal ?
I mean, this happens a lot to any other framework. If you have in mind how many people left CI because it's support for $_GET only then this won't be a mistery anymore.

The only reason that for one is hard to move to other framework is that, like in my case, i had customized everything using CI so rewriting the code for other framework required some work and time, and i know, we don't have enough time because me, like you, i am doing this for money.

It's not a bad idea to have your eyes open to alternatives in the web dev world.